S-NAIL(1)                               BSD General Commands Manual                               S-NAIL(1)

NAME
     S-nail [v14.9.11] — send and receive Internet mail

SYNOPSIS
     s-nail [-DdEFinv~#] [-: spec] [-A account] [:-a attachment:] [:-b bcc-addr:] [:-C "custom: header":]
            [:-c cc-addr:] [-M type | -m file | -q file | -t] [-r from-addr] [:-S var[=value]:]
            [-s subject] [:-X cmd:] [-.] :to-addr: [-- :mta-option:]

     s-nail [-DdEeHiNnRv~#] [-: spec] [-A account] [:-C "custom: header":] [-L spec] [-r from-addr] [:-S
            var[=value]:] [-u user] [:-X cmd:] [-- :mta-option:]
     s-nail [-DdEeHiNnRv~#] [-: spec] [-A account] [:-C "custom: header":] -f [-L spec] [-r from-addr] [:-S
            var[=value]:] [:-X cmd:] [file] [-- :mta-option:]

     s-nail -h | --help
     s-nail -V | --version

DESCRIPTION
           Compatibility  note:  S-nail (S-nail) will wrap up into S-mailx in v15.0 (circa 2020).  Backward
           incompatibility has to be expected – COMMANDS will use Shell-style argument quoting  rules,  for
           example,  and  shell  metacharacters  will  become meaningful.  New and old behaviour is flagged
           [v15-compat] and [no v15-compat], and setting v15-compat, one of the  many  INTERNAL  VARIABLES,
           will  choose  new behaviour when applicable.  [Obsolete] flags what will vanish, and enabling -d
           or -v enables obsoletion warnings.

     S-nail provides a simple and friendly environment for sending and receiving mail.  It is intended to
     provide the functionality of the POSIX mailx(1) command, but is MIME capable and optionally offers ex‐
     tensions for line editing, S/MIME, SMTP and POP3, among others.  S-nail divides incoming mail into its
     constituent messages and allows the user to deal with them in any order.  It offers many COMMANDS and
     INTERNAL VARIABLES for manipulating messages and sending mail.  It provides the user simple editing
     capabilities to ease the composition of outgoing messages, and increasingly powerful and reliable non-
     interactive scripting capabilities.

   Options
     -: spec   Explicitly control which of the Resource files shall be sourced (loaded): if the letter ‘s’
               is (case-insensitively) part of the spec then the system wide /etc/s-nail.rc is sourced,
               likewise the letter ‘u’ controls sourcing of the user's personal ~/.mailrc file, whereas the
               letters ‘-’ and ‘/’ explicitly forbid sourcing of any resource files.  Scripts should use
               this option: to avoid environmental noise they should “detach” from any configuration and
               create a script-specific environment, setting any of the desired INTERNAL VARIABLES via -S
               and running configurating commands via -X.  This option overrides -n.

     -A account
               Executes an account command for the given user email account after program startup is com‐
               plete (all resource files are loaded, any -S setting is being established; only -X commands
               have not been evaluated yet).  Being a special incarnation of defined macros for the purpose
               of bundling longer-lived settings, activating such an email account also switches to the ac‐
               counts primary system mailbox (most likely the inbox).  If the operation fails the program
               will exit if it is used non-interactively, or if any of errexit or posix are set.

     -a file[=input-charset[#output-charset]]
               Attach file to the message (for compose mode opportunities refer to ~@ and ~^).  Filename
               transformations (also see file) will be performed, except that shell variables are not ex‐
               panded.  Shall file not be accessible but contain a ‘=’ character, then anything before the
               last ‘=’ will be used as the filename, anything thereafter as a character set specification.

               If an input character set is specified, but no output character set, then the given input
               character set is fixed as-is, and no conversion will be applied; giving the empty string or
               the special string hyphen-minus ‘-’ will be treated as if ttycharset has been specified (the
               default).

               If an output character set has also been given then the conversion will be performed exactly
               as specified and on-the-fly, not considering the file type and content.  As an exception, if
               the output character set is specified as the empty string or hyphen-minus ‘-’, then the de‐
               fault conversion algorithm (see Character sets) is applied (therefore no conversion is per‐
               formed on-the-fly, file will be MIME-classified and its contents will be inspected first) —
               without support for character set conversions (features does not include the term ‘+iconv’)
               only this argument is supported.

     -B        ([Obsolete]: S-nail will always use line-buffered output, to gain line-buffered input even
               in batch mode enable batch mode via -#.)

     -b addr   Send a blind carbon copy to address, if the setting of expandaddr, one of the INTERNAL
               VARIABLES, allows; the ‘shquote’ expandaddr flag is supported.  The option may be used mul‐
               tiple times.  Also see the section On sending mail, and non-interactive mode.

     -C "field: body"
               Create a custom header which persists for an entire session.  A custom header consists of
               the field name followed by a colon ‘:’ and the field content body, e.g., ‘-C "Blah: Neminem
               laede; imo omnes, quantum potes, juva"’.  Standard header field names cannot be overwritten
               by custom headers.  Runtime adjustable custom headers are available via the variable
               customhdr, and in compose mode ~^, one of the COMMAND ESCAPES, is the most flexible and pow‐
               erful option to manage message headers.  This option may be used multiple times.

     -c addr   Just like -b, except it places the argument in the list of carbon copies.

     -D        ([Option]) Startup with disconnected set.

     -d        Almost enable a sandbox mode with the internal variable debug; the same can be achieved via
               ‘-S debug’ or ‘set debug’.

     -E        set skipemptybody and thus discard messages with an empty message part body.

     -e        Just check if mail is present (in the system inbox or the one specified via -f): if yes, re‐
               turn an exit status of zero, a non-zero value otherwise.  To restrict the set of mails to
               consider in this evaluation a message specification can be added with the option -L.

     -F        Save the message to send in a file named after the local part of the first recipient's ad‐
               dress (instead of in record).

     -f        Read in the contents of the user's secondary mailbox MBOX (or the specified file) for pro‐
               cessing; when S-nail is quit, it writes undeleted messages back to this file (but be aware
               of the hold option).  The optional file argument will undergo some special Filename
               transformations (as via file).  Note that file is not an argument to the flag -f, but is in‐
               stead taken from the command line after option processing has been completed.  In order to
               use a file that starts with a hyphen-minus, prefix with a relative path, as in
               ‘./-hyphenbox.mbox’.

     -H        Display a summary of headers for the given file (depending on -u, inbox or MAIL, or as spec‐
               ified via -f).  A configurable summary view is available via the option -L.  This mode does
               not honour showlast.

     -h        Show a short usage summary.

     -i        set ignore to ignore tty interrupt signals.

     -L spec   Display a summary of headers of all messages that match the given spec in the file found by
               the same algorithm used by -H, then exit.  See the section Specifying messages for the for‐
               mat of spec.  This mode does not honour showlast.

               If the -e option has been given in addition no header summary is produced, but S-nail will
               instead indicate via its exit status whether spec matched any messages (‘0’) or not (‘1’);
               note that any verbose output is suppressed in this mode and must instead be enabled explic‐
               itly (e.g., by using the option -v).

     -M type   Special send mode that will flag standard input with the MIME ‘Content-Type:’ set to the
               given type and use it as the main message body.  [v15 behaviour may differ] Using this op‐
               tion will bypass processing of message-inject-head and message-inject-tail.  Also see -q,
               -m, -t.

     -m file   Special send mode that will MIME classify the specified file and use it as the main message
               body.  [v15 behaviour may differ] Using this option will bypass processing of
               message-inject-head and message-inject-tail.  Also see -q, -M, -t.

     -N        inhibit the initial display of message headers when reading mail or editing a mailbox folder
               by calling unset for the internal variable header.

     -n        Standard flag that inhibits reading the system wide /etc/s-nail.rc upon startup.  The option
               -: allows more control over the startup sequence; also see Resource files.

     -q file   Special send mode that will initialize the message body with the contents of the specified
               file, which may be standard input ‘-’ only in non-interactive context.  Also see -M, -m, -t.

     -R        Any mailbox folder a.k.a. file opened will be in read-only mode.

     -r from-addr
               Whereas the source address that appears in the from header of a message (or in the sender
               header if the former contains multiple addresses) is honoured by the built-in SMTP trans‐
               port, it is not used by a file-based mta (Mail-Transfer-Agent) for the RFC 5321 reverse-path
               used for relaying and delegating a message to its destination(s), for delivery errors etc.,
               but it instead uses the local identity of the initiating user.

               When this command line option is used the given from-addr will be assigned to the internal
               variable from, but in addition the command line option -f from-addr will be passed to a
               file-based mta whenever a message is sent.  Shall from-addr include a user name the address
               components will be separated and the name part will be passed to a file-based mta individu‐
               ally via -F name.

               If an empty string is passed as from-addr then the content of the variable from (or, if that
               contains multiple addresses, sender) will be evaluated and used for this purpose whenever
               the file-based mta is contacted.  By default, without -r that is, neither -f nor -F command
               line options are used when contacting a file-based MTA, unless this automatic deduction is
               enforced by seting the internal variable r-option-implicit.

               Remarks: many default installations and sites disallow overriding the local user identity
               like this unless either the MTA has been configured accordingly or the user is member of a
               group with special privileges.  Passing an invalid address will cause an error.

     -S var[=value]
               set (or, with a prefix string ‘no’, as documented in INTERNAL VARIABLES, unset) variable and
               optionally assign value, if supported; [v15 behaviour may differ] the entire expression is
               evaluated as if specified within dollar-single-quotes (see Shell-style argument quoting) if
               the internal variable v15-compat is set.  If the operation fails the program will exit if
               any of errexit or posix are set.  Settings established via -S cannot be changed from within
               Resource files or an account switch initiated by -A.  They will become mutable again before
               commands registered via -X are executed.

     -s subject
               Specify the subject of the message to be sent.  Newline (NL) and carriage-return (CR) bytes
               are invalid and will be normalized to space (SP) characters.

     -t        The message given (on standard input) is expected to contain, separated from the message
               body by an empty line, one or multiple message headers.  Headers can span multiple consecu‐
               tive lines if follow lines start with any amount of whitespace.  A line starting with the
               number sign ‘#’ in the first column is ignored.  Message recipients can be given via the
               message headers ‘To:’, ‘Cc:’, ‘Bcc:’ or ‘Fcc:’, they will be added to any recipients speci‐
               fied on the command line, and are likewise subject to expandaddr validity checks.  If a mes‐
               sage subject is specified via ‘Subject:’ then it will be used in favour of one given on the
               command line.

               More optional headers are ‘Reply-To:’ (possibly overriding reply-to), ‘Sender:’ (sender),
               ‘From:’ (from and / or option -r).  ‘Message-ID:’, ‘In-Reply-To:’, ‘References:’ and
               ‘Mail-Followup-To:’, by default created automatically dependent on message context, will be
               used if specified (a special address massage will however still occur for the latter).  Any
               other custom header field (also see -C, customhdr and ~^) is passed through entirely un‐
               changed, and in conjunction with the options -~ or -# it is possible to embed COMMAND
               ESCAPES.  Also see -M, -m, -q.

     -u user   Initially read the primary system mailbox of user, appropriate privileges presumed; effec‐
               tively identical to ‘-f %user’.

     -V        Show S-nails version and exit.  The command version will also show the list of features: ‘$
               s-nail -Xversion -Xx’.

     -v        setting the internal variable verbose enables display of some informational context mes‐
               sages.  Using it twice increases the level of verbosity.

     -X cmd    Add the given (or multiple for a multiline argument) cmd to the list of commands to be exe‐
               cuted, as a last step of program startup, before normal operation starts.  This is the only
               possibility to execute commands in non-interactive mode when reading startup files has been
               disabled.  The commands will be evaluated as a unit, just as via source.  Correlates with -#
               and errexit.

     -~        Enable COMMAND ESCAPES in compose mode even in non-interactive use cases.  This can be used
               to, e.g., automatically format the composed message text before sending the message:

                     $ ( echo 'line    one. Word.     Word2.';\
                         echo '~| /usr/bin/fmt -tuw66' ) |\
                       LC_ALL=C s-nail -d~:/ -Sttycharset=utf-8 bob@exam.ple

     -#        Enables batch mode: standard input is made line buffered, the complete set of (interactive)
               commands is available, processing of COMMAND ESCAPES is enabled in compose mode, and diverse
               INTERNAL VARIABLES are adjusted for batch necessities, exactly as if done via set:
               emptystart, noerrexit, noheader, noposix, quiet, sendwait, typescript-mode as well as MAIL,
               MBOX and inbox (the latter three to /dev/null).  The following prepares an email message in
               a batched dry run:

                     $ LC_ALL=C printf 'm bob\n~s ubject\nText\n~.\nx\n' |\
                       LC_ALL=C s-nail -d#:/ -X'alias bob bob@exam.ple'

     -.        This flag forces termination of option processing in order to prevent “option injection”
               (attacks).  It also forcefully puts S-nail into send mode, see On sending mail, and non-
               interactive mode.

     All given to-addr arguments and all receivers established via -b and -c are subject to the checks es‐
     tablished by expandaddr, one of the INTERNAL VARIABLES; they all support the flag ‘shquote’.  If the
     setting of expandargv allows their recognition all mta-option arguments given at the end of the com‐
     mand line after a ‘--’ separator will be passed through to a file-based mta (Mail-Transfer-Agent) and
     persist for the entire session.  expandargv constraints do not apply to the content of mta-arguments.

   A starter
     S-nail is a direct descendant of BSD Mail, itself a successor to the Research UNIX mail which “was
     there from the start” according to HISTORY.  It thus represents the user side of the UNIX mail system,
     whereas the system side (Mail-Transfer-Agent, MTA) was traditionally taken by sendmail(8), and most
     MTAs provide a binary of this name for compatibility purposes.  If the [Option]al SMTP mta is included
     in the features of S-nail then the system side is not a mandatory precondition for mail delivery.

     Because S-nail strives for compliance with POSIX mailx(1) it is likely that some configuration set‐
     tings have to be adjusted before using it is a smooth experience.  (Rather complete configuration ex‐
     amples can be found in the section EXAMPLES.)  The provided global /etc/s-nail.rc (one of the Resource
     files) template bends those standard imposed settings of the INTERNAL VARIABLES a bit towards more
     user friendliness and safety, however.

     For example, it sets hold and keepsave in order to suppress the automatic moving of messages to the
     secondary mailbox MBOX that would otherwise occur (see Message states), and keep to not remove empty
     system MBOX mailbox files (or all empty such files if posix a.k.a. POSIXLY_CORRECT mode has been en‐
     abled) to avoid mangling of file permissions when files eventually get recreated.

     It also enables sendwait in order to synchronize S-nail with the exit status report of the used mta
     when sending mails.  It sets emptystart to enter interactive startup even if the initial mailbox is
     empty, editheaders to allow editing of headers as well as fullnames to not strip down addresses in
     compose mode, and quote to include the message that is being responded to when replying, which is in‐
     dented by an indentprefix that also deviates from standard imposed settings.  mime-counter-evidence is
     fully enabled, too.

     Some random remarks.  The file mode creation mask can be managed explicitly via the variable umask.
     Sufficient system support provided symbolic links will not be followed when files are opened for writ‐
     ing.  Files and shell pipe output can be sourced for evaluation, also during startup from within the
     Resource files.

   On sending mail, and non-interactive mode
     To send a message to one or more people, using a local or built-in mta (Mail-Transfer-Agent) transport
     to actually deliver the generated mail message, S-nail can be invoked with arguments which are the
     names of people to whom the mail will be sent, and the command line options -b and -c can be used to
     add (blind) carbon copy receivers:

           # Via sendmail(1)
           $ s-nail -s ubject -a ttach.txt bill@exam.ple

           # But... try it in an isolated dry-run mode (-d) first
           $ LC_ALL=C s-nail -d -:/ -Ssendwait -Sttycharset=utf8 \
              -b bcc@exam.ple -c cc@exam.ple \
              -Sfullnames -. \
              '(Lovely) Bob <bob@exam.ple>' eric@exam.ple

           # With SMTP
           $ LC_ALL=C s-nail -d -:/ -Sv15-compat -Ssendwait -Sttycharset=utf8 \
               -S mta=smtps://mylogin@exam.ple:465 -Ssmtp-auth=none \
               -S from=scriptreply@exam.ple \
               -a /etc/mail.rc \
               -. eric@exam.ple < /tmp/letter.txt

     If standard input is a terminal rather than the message to be sent, the user is expected to type in
     the message contents.  In this compose mode S-nail treats lines beginning with the character ‘~’ spe‐
     cial – these are so-called COMMAND ESCAPES, which can be used to read in files, process shell com‐
     mands, add and edit attachments and more; e.g., ~v or ~e will start the VISUAL text EDITOR, respec‐
     tively, to revise the message in its current state, ~h allows editing of the most important message
     headers, with the potent ~^ custom headers can be created, for example (more specifically than with -C
     and customhdr).  [Option]ally ~? gives an overview of most other available command escapes.

     The command escape ~. will leave compose mode and send the message once it is completed.  Aborting
     letter composition is possible with either of ~x or ~q, the latter of which will save the message in
     the file denoted by DEAD unless nosave is set.  And unless ignoreeof is set the effect of ~. can also
     be achieved by typing end-of-transmission (EOT) via ‘control-D’ (‘^D’) at the beginning of an empty
     line, and ~q is always reachable by typing end-of-text (ETX) twice via ‘control-C’ (‘^C’).

     A number of ENVIRONMENT and INTERNAL VARIABLES can be used to alter default behavior.  E.g., messages
     are sent asynchronously, without supervision, unless the internal variable sendwait is set, therefore
     send errors will not be recognizable until then.  setting (also via -S) editalong will automatically
     startup an editor when compose mode is entered, and editing of headers additionally to plain body con‐
     tent can be enabled via editheaders: [v15 behaviour may differ] some, but not all headers can be cre‐
     ated, edited or deleted in an editor, then.  askcc and askbcc will cause the user to be prompted ac‐
     tively for (blind) carbon-copy recipients, respectively, and (the default) asksend will request con‐
     firmation whether the message shall be sent.

     The envelope sender address is defined by from, explicitly defining an originating hostname may be de‐
     sirable, especially with the built-in SMTP Mail-Transfer-Agent mta.  Character sets for outgoing mes‐
     sage and MIME part content are configurable via sendcharsets, whereas input data is assumed to be in
     ttycharset.  Message data will be passed over the wire in a mime-encoding.  MIME parts a.k.a. attach‐
     ments need to be assigned a mimetype, usually taken out of The mime.types files.  Saving a copy of
     sent messages in a record mailbox may be desirable – as for most mailbox file targets the value will
     undergo Filename transformations.  Some introductional -d or debug sandbox dry-run tests will prove
     correctness.

     Message recipients (as specified on the command line or defined in ‘To:’, ‘Cc:’ or ‘Bcc:’) are subject
     to alternates filtering, and may not only be email addressees but can also be names of mailboxes and
     even complete shell command pipe specifications.  If the variable expandaddr is not set then only net‐
     work addresses (see mailaddr(7) for a description of mail addresses) and plain user names (including
     MTA aliases) may be used, other types will be filtered out, giving a warning message.  A network ad‐
     dress that contains no domain-, but only a valid local user ‘<name>’ in angle brackets will be auto‐
     matically expanded to a valid address when hostname is set to a non-empty value; setting it to the
     empty value instructs S-nail that the used mta will perform the necessary expansion.  The command
     addrcodec may help to generate standard compliant network addresses.

     If the variable expandaddr is set then an extended set of recipient addresses will be accepted: Any
     name that starts with a vertical bar ‘|’ character specifies a command pipe – the command string fol‐
     lowing the ‘|’ is executed and the message is sent to its standard input; Likewise, any name that con‐
     sists only of hyphen-minus ‘-’ or starts with the character solidus ‘/’ or the character sequence dot
     solidus ‘./’ is treated as a file, regardless of the remaining content.  Any other name which contains
     a commercial at ‘@’ character is a network address; Any other name which starts with a plus sign ‘+’
     character is a mailbox name; Any other name which contains a solidus ‘/’ character but no exclamation
     mark ‘!’ or percent sign ‘%’ character before is also a mailbox name; What remains is treated as a
     network address.

           $ echo bla | s-nail -Sexpandaddr -s test ./mbox.mbox
           $ echo bla | s-nail -Sexpandaddr -s test '|cat >> ./mbox.mbox'
           $ echo safe | LC_ALL=C \
               s-nail -:/ -Sv15-compat -Ssendwait -Sttycharset=utf8 \
                 -Sexpandaddr=fail,-all,+addr,failinvaddr -s test \
                 -. bob@exam.ple

     To create file-carbon-copies the special recipient header ‘Fcc:’ may be used as often as desired.  Its
     entire value (or body in standard terms) is interpreted as a file target, after having been subject to
     Filename transformations.  Beside using the command escape ~^ (to create a ‘Fcc’ header) this is the
     only way to create a file-carbon-copy without introducing an ambiguity regarding the interpretation of
     the address, e.g., to use file names with leading vertical bars or commercial ats.  Like all other re‐
     cipients ‘Fcc:’ is subject to the checks of expandaddr.

     It is possible to create personal distribution lists via the alias command, so that, for instance, the
     user can send mail to ‘cohorts’ and have it go to a group of people.  Different to the alias mechanism
     of a local mta, which is often tracked in a file /etc/aliases, documented in aliases(5), and the names
     of which are subject to the ‘name’ constraint of expandaddr, personal aliases will be expanded by
     S-nail before the message is sent.  They are thus a convenient alternative to specifying each ad‐
     dressee by itself, correlate with the active set of alternates, and are subject to metoo filtering.

           ? alias  cohorts  bill jkf mark kridle@ucbcory ~/cohorts.mbox
           ? alias  mark  mark@exam.ple

     on-compose-enter, on-compose-leave and on-compose-cleanup hook variables may be set to defined macros
     to automatically adjust some settings dependent on receiver, sender or subject contexts, and via the
     on-compose-splice as well as on-compose-splice-shell variables, the former also to be set to a defined
     macro, increasingly powerful mechanisms to perform automated message adjustments, including signature
     creation, are available.  ([v15 behaviour may differ] These hooks work for commands which newly create
     messages, namely forward, mail, reply and variants; resend and Resend for now provide only the hooks
     on-resend-enter and on-resend-cleanup.)

     For the purpose of arranging a complete environment of settings that can be switched to with a single
     command or command line option there are accounts.  Alternatively it is also possible to use a flat
     configuration, making use of so-called variable chains which automatically pick ‘USER@HOST’ or ‘HOST’
     context-dependent variable variants: for example addressing ‘File pop3://yaa@exam.ple’ would find
     pop3-no-apop-yaa@exam.ple, pop3-no-apop-exam.ple and pop3-no-apop in order.  See On URL syntax and
     credential lookup and INTERNAL VARIABLES.

     To avoid environmental noise scripts should “detach” S-nail from any configuration files and create a
     script-local environment, ideally with the command line options -: to disable any configuration file
     in conjunction with repetitions of -S to specify variables:

           $ env LC_ALL=C s-nail -:/ \
               -Sv15-compat -Ssendwait -Sttycharset=utf-8 \
               -Sexpandaddr=fail,-all,failinvaddr \
               -S mta=smtps://mylogin@exam.ple:465 -Ssmtp-auth=login \
               -S from=scriptreply@exam.ple \
               -s 'Subject to go' -a attachment_file \
               -Sfullnames -. \
               'Recipient 1 <rec1@exam.ple>' rec2@exam.ple \
               < content_file

     As shown, scripts can “fake” a locale environment, the above specifies the all-compatible 7-bit clean
     LC_ALL “C”, but will nonetheless take and send UTF-8 in the message text by using ttycharset.  In in‐
     teractive mode, which is introduced in the next section, messages can be sent by calling the mail com‐
     mand with a list of recipient addresses:

           $ s-nail -d -Squiet -Semptystart
           "/var/spool/mail/user": 0 messages
           ? mail "Recipient 1 <rec1@exam.ple>", rec2@exam.ple
           ...
           ? # Will do the right thing (tm)
           ? m rec1@exam.ple rec2@exam.ple

   On reading mail, and interactive mode
     When invoked without addressees S-nail enters interactive mode in which mails may be read.  When used
     like that the user's system inbox (for more on mailbox types please see the command file) is read in
     and a one line header of each message therein is displayed if the variable header is set.  The visual
     style of this summary of headers can be adjusted through the variable headline and the possible sort‐
     ing criterion via autosort.  Scrolling through screenfuls of headers can be performed with the command
     z.  If the initially opened mailbox is empty S-nail will instead exit immediately (after displaying a
     message) unless the variable emptystart is set.

     At the prompt the command list will give a listing of all available commands and help will [Op‐
     tion]ally give a summary of some common ones.  If the [Option]al documentation strings are available
     (see features) one can type ‘help X’ (or ‘?X’) and see the actual expansion of ‘X’ and what its pur‐
     pose is, i.e., commands can be abbreviated (note that POSIX defines some abbreviations, so that the
     alphabetical order of commands does not necessarily relate to the abbreviations; it is however possi‐
     ble to define overwrites with commandalias).  These commands can also produce a more verbose output.

     Messages are given numbers (starting at 1) which uniquely identify messages; the current message – the
     “dot” – will either be the first new message, or the first unread message, or the first message of the
     mailbox; the internal variable showlast will instead cause usage of the last message for this purpose.
     The command headers will display a screenful of header summaries containing the “dot”, whereas from
     will display only the summaries of the given messages, defaulting to the “dot”.

     Message content can be displayed with the command type (‘t’, alias print).  Here the variable crt con‐
     trols whether and when S-nail will use the configured PAGER for display instead of directly writing to
     the user terminal screen, the sole difference to the command more, which will always use the PAGER.
     The command top will instead only show the first toplines of a message (maybe even compressed if
     topsqueeze is set).  Message display experience may improve by setting and adjusting
     mime-counter-evidence, and also see HTML mail and MIME attachments.

     By default the current message (“dot”) is displayed, but like with many other commands it is possible
     to give a fancy message specification (see Specifying messages), e.g., ‘t:u’ will display all unread
     messages, ‘t.’ will display the “dot”, ‘t 1 5’ will type the messages 1 and 5, ‘t 1-5’ will type the
     messages 1 through 5, and ‘t-’ and ‘t+’ will display the previous and the next message, respectively.
     The command search (a more substantial alias for from) will display a header summary of the given mes‐
     sage specification list instead of their content, e.g., the following will search for subjects:

           ? from '@Some subject to search for'

     In the default setup all header fields of a message will be typed, but fields can be white- or black‐
     listed for a variety of applications by using the command headerpick, e.g., to restrict their display
     to a very restricted set for type: ‘headerpick type retain from to cc subject’.  In order to display
     all header fields of a message regardless of currently active ignore or retain lists, use the commands
     Type and Top; Show will show the raw message content.  Note that historically the global /etc/
     s-nail.rc not only adjusts the list of displayed headers, but also sets crt.  ([v15 behaviour may dif‐
     fer] A yet somewhat restricted) Reliable scriptable message inspection is available via digmsg.

     Dependent upon the configuration a line editor (see the section On terminal control and line editor)
     aims at making the user experience with the many COMMANDS a bit nicer.  When reading the system inbox,
     or when -f (or file) specified a mailbox explicitly prefixed with the special ‘%:’ modifier (to propa‐
     gate it to a primary system mailbox), then messages which have been read (see Message states) will be
     automatically moved to a secondary mailbox, the user's MBOX file, when the mailbox is left, either by
     changing the active mailbox or by quitting S-nail – this automatic moving from a system- or primary-
     to the secondary mailbox is not performed when the variable hold is set.  Messages can also be explic‐
     itly moved to other mailboxes, whereas copy keeps the original message.  write can be used to write
     out data content of specific parts of messages.

     After examining a message the user can reply ‘r’ to the sender and all recipients (which will also be
     placed in ‘To:’ unless recipients-in-cc is set), or Reply ‘R’ exclusively to the sender(s).  The com‐
     mand Lreply knows how to apply a special addressee massage, see Mailing lists.  Dependent on the pres‐
     ence and value of quote the message being replied to will be included in a quoted form.  forwarding a
     message will allow editing the new message: the original message will be contained in the message
     body, adjusted according to headerpick.  It is possible to resend or Resend messages: the former will
     add a series of ‘Resent-’ headers, whereas the latter will not; different to newly created messages
     editing is not possible and no copy will be saved even with record unless the additional variable
     record-resent is set.  When sending, replying or forwarding messages comments and full names will be
     stripped from recipient addresses unless the internal variable fullnames is set.

     Of course messages can be delete ‘d’, and they can spring into existence again via undelete, or when
     the S-nail session is ended via the exit or xit commands to perform a quick program termation.  To end
     a mail processing session regulary and perform a full program exit one may issue the command quit.  It
     will, among others, move read messages to the secondary mailbox MBOX as necessary, discard deleted
     messages in the current mailbox, and update the [Option]al (see features) line editor history-file.

   HTML mail and MIME attachments
     Messages which are HTML-only become more and more common, and of course many messages come bundled
     with a bouquet of MIME (Multipurpose Internet Mail Extensions) parts.  To get a notion of MIME types
     S-nail has a default set of types built-in, onto which the content of The mime.types files will be
     added (as configured and allowed by mimetypes-load-control).  Types can also become registered with
     the command mimetype.  To improve interaction with faulty MIME part declarations which are often seen
     in real-life messages, setting mime-counter-evidence will allow verification of the given assertion,
     and possible provision of an alternative, better MIME type.

     Whereas S-nail [Option]ally supports a simple HTML-to-text filter for displaying HTML messages, it
     cannot handle MIME types other than plain text itself.  Instead programs need to become registered to
     deal with specific MIME types or file extensions.  These programs may either prepare plain text ver‐
     sions of their input in order to enable S-nail to integrate their output neatlessly in its own message
     visualization (a mode which is called copiousoutput), or display the content themselves, for example
     in an external graphical window: such handlers will only be considered by and for the command
     mimeview.

     To install a handler program for a specific MIME type an according pipe-TYPE/SUBTYPE variable needs to
     be set; to instead define a handler for a specific file extension the respective pipe-EXTENSION vari‐
     able can be used – these handlers take precedence.  [Option]ally S-nail supports mail user agent con‐
     figuration as defined in RFC 1524; this mechanism (see The Mailcap files) will be queried for display
     or quote handlers if none of the former two did; it will be the sole source for handlers of other pur‐
     pose.  A last source for handlers is the MIME type definition itself, if a type-marker has been regis‐
     tered with the command mimetype, which many of the built-in MIME types do.

     For example, to display a HTML message inline (converted to a more fancy plain text representation
     than the built-in filter is capable to produce) with either of the text-mode browsers lynx(1) or
     elinks(1), teach S-nail about MathML documents and make it display them as plain text, and to open PDF
     attachments in an external PDF viewer, asynchronously and with some other magic attached:

           ? if [ "$features" !% +filter-html-tagsoup ]
           ?   #set pipe-text/html='@* elinks -force-html -dump 1'
           ?   set pipe-text/html='@* lynx -stdin -dump -force_html'
           ?   # Display HTML as plain text instead
           ?   #set pipe-text/html=@
           ? endif
           ? mimetype @ application/mathml+xml mathml
           ? wysh set pipe-application/pdf='@&=@ \
               trap "rm -f \"${MAILX_FILENAME_TEMPORARY}\"" EXIT;\
               trap "trap \"\" INT QUIT TERM; exit 1" INT QUIT TERM;\
               mupdf "${MAILX_FILENAME_TEMPORARY}"'

   Mailing lists
     S-nail offers some support to ease handling of mailing lists.  The command mlist promotes all given
     arguments to known mailing lists, and mlsubscribe sets their subscription attribute, creating them
     first as necessary.  (On the other hand unmlsubscribe does not unmlist automatically, but only resets
     the subscription attribute.)  Using the commands without arguments will show (a subset of) all cur‐
     rently defined mailing lists.  The headline format ‘%T’ can be used to mark out messages with config‐
     ured list addresses in the display of headers.

     If the [Option]al regular expression support is available a mailing list specification that contains
     any of the “magical” regular expression characters ‘^[]*+?|$’ (see re_format(7)) will be interpreted
     as one, which allows matching of many addresses with a single expression.  However, all fully quali‐
     fied list addresses are matched via a fast dictionary, whereas expressions are placed in (a) list(s)
     which is (are) matched sequentially.

           ? set followup-to followup-to-honour=ask-yes \
               reply-to-honour=ask-yes
           ? wysh mlist a1@b1.c1 a2@b2.c2 '.*@lists\.c3$'
           ? mlsubscribe a4@b4.c4 exact@lists.c3

     The variable followup-to-honour will ensure that a ‘Mail-Followup-To:’ header is honoured when the
     message is being replied to (via reply and Lreply) and followup-to controls whether this header is
     created when sending mails; it will be created automatically for a couple of reasons, too, like when
     the special “mailing list specific” respond command Lreply is used, when reply is used to respond to a
     message with its ‘Mail-Followup-To:’ being honoured etc.

     A difference in between the handling of known and subscribed lists is that the address of the user is
     usually not part of a generated ‘Mail-Followup-To:’ when addressing the latter, whereas it is for the
     former kind of lists.  Usually because there are exceptions: say, if multiple lists are addressed and
     not all of them are subscribed lists.

     For convenience S-nail will, temporarily, automatically add a list address that is presented in the
     ‘List-Post:’ header of a message that is being responded to to the list of known mailing lists.  Shall
     that header have existed S-nail will instead, dependent on the variable reply-to-honour, use an also
     set ‘Reply-To:’ for this purpose (if it provides a single address which resides on the same domain as
     what is stated in ‘List-Post:’) in order to accept a list administrator's wish that is supposed to
     have been manifested like that.

   Signed and encrypted messages with S/MIME
     [Option] S/MIME provides two central mechanisms: message signing and message encryption.  A signed
     message contains some data in addition to the regular text.  The data can be used to verify that the
     message has been sent using a valid certificate, that the sender address matches that in the certifi‐
     cate, and that the message text has not been altered.  Signing a message does not change its regular
     text; it can be read regardless of whether the recipients software is able to handle S/MIME.  It is
     thus usually possible to sign all outgoing messages if so desired.

     Encryption, in contrast, makes the message text invisible for all people except those who have access
     to the secret decryption key.  To encrypt a message, the specific recipients public encryption key
     must be known.  It is therefore not possible to send encrypted mail to people unless their key has
     been retrieved from either previous communication or public key directories.  Because signing is per‐
     formed with private keys, and encryption with public keys, messages should always be signed before be‐
     coming encrypted.

     A central concept to S/MIME is that of the certification authority (CA).  A CA is a trusted institu‐
     tion that issues certificates.  For each of these certificates it can be verified that it really orig‐
     inates from the CA, provided that the CA's own certificate is previously known.  A set of CA certifi‐
     cates is usually delivered and installed together with the cryptographical library that is used on the
     local system.  Therefore reasonable security for S/MIME on the Internet is provided if the source that
     provides that library installation is trusted.  It is also possible to use a specific pool of trusted
     certificates.  If this is desired, smime-ca-no-defaults should be set to avoid using the default cer‐
     tificate pool, and smime-ca-file and/or smime-ca-dir should be pointed to a trusted pool of certifi‐
     cates.  A certificate cannot be more secure than the method its CA certificate has been retrieved
     with.

     This trusted pool of certificates is used by the command verify to ensure that the given S/MIME mes‐
     sages can be trusted.  If so, verified sender certificates that were embedded in signed messages can
     be saved locally with the command certsave, and used by S-nail to encrypt further communication with
     these senders:

           ? certsave FILENAME
           ? set smime-encrypt-USER@HOST=FILENAME \
               smime-cipher-USER@HOST=AES256

     To sign outgoing messages, in order to allow receivers to verify the origin of these messages, a per‐
     sonal S/MIME certificate is required.  S-nail supports password-protected personal certificates (and
     keys), see smime-sign-cert.  The section On URL syntax and credential lookup gives an overview of the
     possible sources of user credentials, and S/MIME step by step shows examplarily how a private S/MIME
     certificate can be obtained.  In general, if such a private key plus certificate “pair” is available,
     all that needs to be done is to set some variables:

           ? set smime-sign-cert=ME@exam.ple.paired \
               smime-sign-digest=SHA512 \
               smime-sign

     Variables of interest for S/MIME in general are smime-ca-dir, smime-ca-file, smime-ca-flags,
     smime-ca-no-defaults, smime-crl-dir, smime-crl-file.  For S/MIME signing of interest are smime-sign,
     smime-sign-cert, smime-sign-include-certs and smime-sign-digest.  Additional variables of interest for
     S/MIME en- and decryption: smime-cipher and smime-encrypt-USER@HOST.

     [v15 behaviour may differ] Note that neither S/MIME signing nor encryption applies to message subjects
     or other header fields yet.  Thus they may not contain sensitive information for encrypted messages,
     and cannot be trusted even if the message content has been verified.  When sending signed messages, it
     is recommended to repeat any important header information in the message text.

   On URL syntax and credential lookup
     [v15-compat] For accessing protocol-specific resources usage of Uniform Resource Locators (URL, RFC
     1738) has become omnipresent.  S-nail expects and understands URLs in the following form; parts in
     brackets ‘[]’ denote optional parts, optional either because there also exist other ways to define the
     information in question or because support of the part is protocol-specific, e.g., ‘/path’ is used by
     the [Option]al Maildir directory and the IMAP protocol, but not by POP3; If any of ‘USER’ and
     ‘PASSWORD’ are specified they must be given in URL percent encoded form (RFC 3986; the command
     urlcodec may be helpful):

           PROTOCOL://[USER[:PASSWORD]@]server[:port][/path]

     Note that these S-nail URLs most often do not conform to any real standard, but instead represent a
     normalized variant of RFC 1738 – they are not used in data exchange but only meant as a compact, easy-
     to-use way of defining and representing information in a well-known notation.

     Many internal variables of S-nail exist in multiple versions, called variable chains for the rest of
     this document: the plain ‘variable’ as well as ‘variable-HOST’ and ‘variable-USER@HOST’.  Here ‘HOST’
     indeed means ‘server:port’ if a ‘port’ had been specified in the respective URL, otherwise it refers
     to the plain ‘server’.  Also, ‘USER’ is not truly the ‘USER’ that had been found when doing the user
     chain lookup as is described below, i.e., this ‘USER’ will never be in URL percent encoded form,
     whether it came from an URL or not; i.e., variable chain name extensions of INTERNAL VARIABLES must
     not be URL percent encoded.

     For example, whether an hypothetical URL ‘smtp://hey%3Ayou@our.house’ had been given that includes a
     user, or whether the URL was ‘smtp://our.house’ and the user had been found differently, to lookup the
     variable chain smtp-use-starttls S-nail first looks for whether ‘smtp-use-starttls-hey:you@our.house’
     is defined, then whether ‘smtp-use-starttls-our.house’ exists before finally ending up looking at the
     plain variable itself.

     S-nail obeys the following logic scheme when dealing with the necessary credential information of an
     account:

     •   If no ‘USER’ has been given in the URL the variables user-HOST and user are looked up.  If no such
         variable(s) can be found then S-nail will, when enforced by the [Option]al variables
         netrc-lookup-HOST or netrc-lookup, search The .netrc file of the user for a ‘HOST’ specific entry
         which provides a ‘login’ name: this lookup will only succeed if unambiguous (one possible matching
         entry for ‘HOST’).

         If there is still no ‘USER’ then S-nail will fall back to the user who is supposed to run S-nail,
         the identity of which has been fixated during S-nail startup and is known to be a valid user on
         the current host.

     •   Authentication: unless otherwise noted this will lookup the PROTOCOL-auth-USER@HOST,
         PROTOCOL-auth-HOST, PROTOCOL-auth variable chain, falling back to a protocol-specific default
         should this have no success.

     •   If no ‘PASSWORD’ has been given in the URL, then if the ‘USER’ has been found through the [Op‐
         tion]al netrc-lookup that may have already provided the password, too.  Otherwise the variable
         chain password-USER@HOST, password-HOST, password is looked up and used if existent.

         Afterwards the complete [Option]al variable chain netrc-lookup-USER@HOST, netrc-lookup-HOST,
         netrc-lookup is looked up.  If set, the netrc cache is searched for a password only (multiple user
         accounts for a single machine may exist as well as a fallback entry without user but with a pass‐
         word).

         If at that point there is still no password available, but the (protocols') chosen authentication
         type requires a password, then in interactive mode the user will be prompted on the terminal.

     Note: S/MIME verification works relative to the values found in the ‘From:’ (or ‘Sender:’) header
     field(s), which means that the values of smime-sign, smime-sign-cert, smime-sign-include-certs and
     smime-sign-digest will not be looked up using the ‘USER’ and ‘HOST’ chains from above but instead use
     the corresponding values from the message that is being worked on.  In unusual cases multiple and dif‐
     ferent ‘USER’ and ‘HOST’ combinations may therefore be involved – on the other hand those unusual
     cases become possible.  The usual case is as short as:

           set mta=smtp://USER:PASS@HOST smtp-use-starttls \
               smime-sign smime-sign-cert=+smime.pair

     The section EXAMPLES contains complete example configurations.

   Encrypted network communication
     SSL (Secure Sockets Layer) a.k.a. its successor TLS (Transport Layer Security) are protocols which aid
     in securing communication by providing a safely initiated and encrypted network connection.  A central
     concept of TLS is that of certificates: as part of each network connection setup a (set of) certifi‐
     cates will be exchanged, and by using those the identity of the network peer can be cryptographically
     verified; if possible the TLS/SNI (ServerNameIndication) extension will be enabled in order to allow
     servers fine-grained control over the certificates being used.  TLS works by using a locally installed
     pool of trusted certificates, and verifying the connection peer succeeds if that provides a certifi‐
     cate which has been issued or is trusted by any certificate in the trusted local pool.

     The local pool of trusted so-called CA (Certification Authority) certificates is usually delivered
     with the used TLS library, and will be selected automatically.  It is also possible to use a specific
     pool of trusted certificates.  If this is desired, tls-ca-no-defaults should be set to avoid using the
     default certificate pool, and tls-ca-file and/or (with special preparation) tls-ca-dir should be
     pointed to a trusted pool of certificates.  A certificate cannot be more secure than the method its CA
     certificate has been retrieved with.  For inspection or other purposes, the certificate of a server
     (as seen when connecting to it) can be fetched like this:

           $ </dev/null openssl s_client -showcerts -connect \
               the-server.example:pop3s 2>&1 | tee log.txt

     S-nail also supports a mode of operation in which certificates are not at all matched against a local
     pool of CA certificates.  Instead a message digest will be calculated for the certificate presented by
     the connection peer, and be compared against tls-fingerprint (a variable chain that picks up
     ‘USER@HOST’ or ‘HOST’ context-dependent variable variants), and the connection will succeed if the
     calculated digest equals the expected one.  The used message digest can be configured via (the chain)
     tls-fingerprint-digest.  The command tls may be helpful.

     It depends on the used protocol whether encrypted communication is possible, and which configuration
     steps have to be taken to enable it.  Some protocols, e.g., POP3S, are implicitly encrypted, others,
     like POP3, can upgrade a plain text connection if so requested.  For example, to use the ‘STLS’ that
     POP3 offers (a member of) the variable (chain) pop3-use-starttls needs to be set:

           shortcut encpop1 pop3s://pop1.exam.ple

           shortcut encpop2 pop3://pop2.exam.ple
           set pop3-use-starttls-pop2.exam.ple

           set mta=smtps://smtp.exam.ple:465
           set mta=smtp://smtp.exam.ple smtp-use-starttls

     Normally that is all there is to do, given that TLS libraries try to provide safe defaults, plenty of
     knobs however exist to adjust settings.  For example certificate verification settings can be fine-
     tuned via tls-ca-flags, and the TLS configuration basics are accessible via tls-config-pairs, for ex‐
     ample to specify the allowed protocols or cipher lists that a communication channel may use.  In the
     past hints on how to restrict the set of protocols to highly secure ones were indicated, but as of the
     time of this writing the list of protocols or ciphers may need to become relaxed in order to be able
     to connect to some servers; the following example allows connecting to a “Lion” that uses OpenSSL
     0.9.8za from June 2014 (refer to INTERNAL VARIABLES for more on variable chains):

           wysh set tls-config-pairs-lion@exam.ple='MinProtocol=TLSv1.1,\
               CipherString=TLSv1.2:!aNULL:!eNULL:\
                 ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:\
                 DHE-RSA-AES256-SHA:@STRENGTH'

     The OpenSSL program ciphers(1) can be used and should be referred to when creating a custom cipher
     list.  Variables of interest for TLS in general are tls-ca-dir, tls-ca-file, tls-ca-flags,
     tls-ca-no-defaults, tls-config-file, tls-config-module, tls-config-pairs, tls-crl-dir, tls-crl-file,
     tls-rand-file as well as tls-verify.

   Character sets
     [Option] S-nail detects the character set of the terminal by using mechanisms that are controlled by
     the LC_CTYPE environment variable (in fact LC_ALL, LC_CTYPE, LANG, in that order, see there).  The in‐
     ternal variable ttycharset will be set to the detected terminal character set accordingly, and will
     thus show up in the output of commands like, e.g., set and varshow.

     However, the user may give ttycharset a value during startup, making it possible to send mail in a
     completely “faked” locale environment, an option which can be used to generate and send, e.g., 8-bit
     UTF-8 input data in a pure 7-bit US-ASCII ‘LC_ALL=C’ environment (an example of this can be found in
     the section On sending mail, and non-interactive mode).  Changing the value does not mean much beside
     that, because several aspects of the real character set are implied by the locale environment of the
     system, which stays unaffected by ttycharset.

     Messages and attachments which consist of 7-bit clean data will be classified as consisting of
     charset-7bit character data.  This is a problem if the ttycharset character set is a multibyte charac‐
     ter set that is also 7-bit clean.  For example, the Japanese character set ISO-2022-JP is 7-bit clean
     but capable to encode the rich set of Japanese Kanji, Hiragana and Katakana characters: in order to
     notify receivers of this character set the mail message must be MIME encoded so that the character set
     ISO-2022-JP can be advertised!  To achieve this, the variable charset-7bit must be set to ISO-2022-JP.
     (Today a better approach regarding email is the usage of UTF-8, which uses 8-bit bytes for non-US-
     ASCII data.)

     If the [Option]al character set conversion capabilities are not available (features does not include
     the term ‘+iconv’), then ttycharset will be the only supported character set, it is simply assumed
     that it can be used to exchange 8-bit messages (over the wire an intermediate, configurable
     mime-encoding may be applied), and the rest of this section does not apply; it may however still be
     necessary to explicitly set it if automatic detection fails, since in that case it defaults to LATIN1
     a.k.a. ISO-8859-1 unless the operating system environment is known to always and exclusively support
     UTF-8 locales.

     [Option] When reading messages, their text is converted into ttycharset as necessary in order to dis‐
     play them on the user's terminal.  Unprintable characters and invalid byte sequences are detected and
     replaced by proper substitution characters.  Character set mappings for source character sets can be
     established with the command charsetalias, which may be handy to work around faulty character set cat‐
     alogues (e.g., to add a missing LATIN1 to ISO-8859-1 mapping), or to enforce treatment of one charac‐
     ter set as another one (e.g., to interpret LATIN1 as CP1252).  Also see charset-unknown-8bit to deal
     with another hairy aspect of message interpretation.

     When sending messages their parts and attachments are classified.  Whereas no character set conversion
     is performed on those parts which appear to be binary data, the character set being used must be de‐
     clared within the MIME header of an outgoing text part if it contains characters that do not conform
     to the set of characters that are allowed by the email standards.  Permissible values for character
     sets used in outgoing messages can be declared using the sendcharsets variable, and charset-8bit,
     which defines a catch-all last-resort fallback character set that is implicitly appended to the list
     of character sets in sendcharsets.

     When replying to a message and the variable reply-in-same-charset is set, then the character set of
     the message being replied to is tried first (still being a subject of charsetalias).  And it is also
     possible to make S-nail work even more closely related to the current locale setting automatically by
     using the variable sendcharsets-else-ttycharset, please see there for more information.

     All the specified character sets are tried in order unless the conversion of the part or attachment
     succeeds.  If none of the tried (8-bit) character sets is capable to represent the content of the part
     or attachment, then the message will not be send and its text will optionally be saved in DEAD.  In
     general, if a message saying “cannot convert from a to b” appears, either some characters are not ap‐
     propriate for the currently selected (terminal) character set, or the needed conversion is not sup‐
     ported by the system.  In the first case, it is necessary to set an appropriate LC_CTYPE locale and/or
     the variable ttycharset.

     The best results are usually achieved when S-nail is run in a UTF-8 locale on an UTF-8 capable termi‐
     nal, in which case the full Unicode spectrum of characters is available.  In this setup characters
     from various countries can be displayed, while it is still possible to use more simple character sets
     for sending to retain maximum compatibility with older mail clients.

     On the other hand the POSIX standard defines a locale-independent 7-bit “portable character set” that
     should be used when overall portability is an issue, the even more restricted subset named “portable
     filename character set” consists of A-Z, a-z, 0-9, period ‘.’, underscore ‘_’ and hyphen-minus ‘-’.

   Message states
     S-nail differentiates in between several message states; the current state will be reflected in the
     summary of headers if the attrlist of the configured headline allows, and Specifying messages depen‐
     dent on their state is possible.  When operating on the system inbox, or in any other primary system
     mailbox, special actions, like the automatic moving of messages to the secondary mailbox MBOX, may be
     applied when the mailbox is left (also implicitly by program termination, unless the command exit was
     used) – however, because this may be irritating to users which are used to “more modern” mail-user-
     agents, the provided global /etc/s-nail.rc template sets the internal hold and keepsave variables in
     order to suppress this behaviour.

     ‘new’     Message has neither been viewed nor moved to any other state.  Such messages are retained
               even in the primary system mailbox.

     ‘unread’  Message has neither been viewed nor moved to any other state, but the message was present
               already when the mailbox has been opened last: Such messages are retained even in the
               primary system mailbox.

     ‘read’    The message has been processed by one of the following commands: ~f, ~m, ~F, ~M, copy, mbox,
               next, pipe, Print, print, top, Type, type, undelete.  The commands dp and dt will always try
               to automatically “step” and type the “next” logical message, and may thus mark multiple mes‐
               sages as read, the delete command will do so if the internal variable autoprint is set.

               Except when the exit command is used, messages that are in a primary system mailbox and are
               in ‘read’ state when the mailbox is left will be saved in the secondary mailbox MBOX unless
               the internal variable hold it set.

     ‘deleted’ The message has been processed by one of the following commands: delete, dp, dt.  Only
               undelete can be used to access such messages.

     ‘preserved’ The message has been processed by a preserve command and it will be retained in its cur‐
               rent location.

     ‘saved’   The message has been processed by one of the following commands: save or write.  Unless when
               the exit command is used, messages that are in a primary system mailbox and are in ‘saved’
               state when the mailbox is left will be deleted; they will be saved in the secondary mailbox
               MBOX when the internal variable keepsave is set.

     In addition to these message states, flags which otherwise have no technical meaning in the mail sys‐
     tem except allowing special ways of addressing them when Specifying messages can be set on messages.
     These flags are saved with messages and are thus persistent, and are portable between a set of widely
     used MUAs.

     answered  Mark messages as having been answered.

     draft     Mark messages as being a draft.

     flag      Mark messages which need special attention.

   Specifying messages
     [Only new quoting rules] Commands which take Message list arguments, such as from a.k.a. search, type
     and delete, can be given a list of message numbers as arguments to apply to a number of messages at
     once.  Thus ‘delete 1 2’ deletes messages 1 and 2, whereas ‘delete 1-5’ will delete the messages 1
     through 5.  In sorted or threaded mode (see the sort command), ‘delete 1-5’ will delete the messages
     that are located between (and including) messages 1 through 5 in the sorted/threaded order, as shown
     in the headers summary.  The following special message names exist:

     .         The current message, the so-called “dot”.

     ;         The message that was previously the current message; needs to be quoted.

     ,         The parent message of the current message, that is the message with the Message-ID given in
               the ‘In-Reply-To:’ field or the last entry of the ‘References:’ field of the current mes‐
               sage.

     -         The previous undeleted message, or the previous deleted message for the undelete command; In
               sorted or ‘thread’ed mode, the previous such message in the according order.

     +         The next undeleted message, or the next deleted message for the undelete command; In sorted
               or ‘thread’ed mode, the next such message in the according order.

     ^         The first undeleted message, or the first deleted message for the undelete command; In
               sorted or ‘thread’ed mode, the first such message in the according order.

     $         The last message; In sorted or ‘thread’ed mode, the last such message in the according or‐
               der.  Needs to be quoted.

     &x        In ‘thread’ed sort mode, selects the message addressed with x, where x is any other message
               specification, and all messages from the thread that begins at it.  Otherwise it is identi‐
               cal to x.  If x is omitted, the thread beginning with the current message is selected.

     *         All messages.

     `         All messages that were included in the Message list arguments of the previous command; needs
               to be quoted.

     x-y       An inclusive range of message numbers.  Selectors that may also be used as endpoints include
               any of .;-+^$.

     address   A case-insensitive “any substring matches” search against the ‘From:’ header, which will
               match addresses (too) even if showname is set (and POSIX says “any address as shown in a
               header summary shall be matchable in this form”); However, if the allnet variable is set,
               only the local part of the address is evaluated for the comparison, not ignoring case, and
               the setting of showname is completely ignored.  For finer control and match boundaries use
               the ‘@’ search expression.

     /string   All messages that contain string in the subject field (case ignored according to locale).
               See also the searchheaders variable.  If string is empty, the string from the previous spec‐
               ification of that type is used again.

     [@name-list]@expr
               All messages that contain the given case-insensitive search expression;  If the [Option]al
               regular expression support is available expr will be interpreted as (an extended) one if any
               of the “magical” regular expression characters ‘^[]*+?|$’ is seen (see re_format(7)).  If
               the optional @name-list part is missing the search is restricted to the subject field body,
               but otherwise name-list specifies a comma-separated list of header fields to search, e.g.,

                     '@to,from,cc@Someone i ought to know'

               In order to search for a string that includes a ‘@’ (commercial at) character the name-list
               is effectively non-optional, but may be given as the empty string.  Also, specifying an
               empty search expression will effectively test for existence of the given header fields.
               Some special header fields may be abbreviated: ‘f’, ‘t’, ‘c’, ‘b’ and ‘s’ will match ‘From’,
               ‘To’, ‘Cc’, ‘Bcc’ and ‘Subject’, respectively and case-insensitively.  [Option]ally, and
               just like expr, name-list will be interpreted as (an extended) regular expression if any of
               the “magical” regular expression characters is seen.

               The special names ‘header’ or ‘<’ can be used to search in (all of) the header(s) of the
               message, and the special names ‘body’ or ‘>’ and ‘text’ or ‘=’ will perform full text
               searches – whereas the former searches only the body, the latter also searches the message
               header ([v15 behaviour may differ] this mode yet brute force searches over the entire de‐
               coded content of messages, including administrativa strings).

               This specification performs full text comparison, but even with regular expression support
               it is almost impossible to write a search expression that safely matches only a specific ad‐
               dress domain.  To request that the body content of the header is treated as a list of ad‐
               dresses, and to strip those down to the plain email address which the search expression is
               to be matched against, prefix the effective name-list with a tilde ‘~’:

                     '@~f,c@@a\.safe\.domain\.match$'

     :c        All messages of state or with matching condition ‘c’, where ‘c’ is one or multiple of the
               following colon modifiers:

               a         answered messages (cf. the variable markanswered).
               d         ‘deleted’ messages (for the undelete and from commands only).
               f         flagged messages.
               L         Messages with receivers that match mlsubscribed addresses.
               l         Messages with receivers that match mlisted addresses.
               n         ‘new’ messages.
               o         Old messages (any not in state ‘read’ or ‘new’).
               r         ‘read’ messages.
               S         [Option] Messages with unsure spam classification (see Handling spam).
               s         [Option] Messages classified as spam.
               t         Messages marked as draft.
               u         ‘unread’ messages.

     [Option] IMAP-style SEARCH expressions may also be used.  These consist of keywords and criterions,
     and because Message list arguments are split into tokens according to Shell-style argument quoting it
     is necessary to quote the entire IMAP search expression in order to ensure that it remains a single
     token.  This addressing mode is available with all types of mailbox folders; S-nail will perform the
     search locally as necessary.  Strings must be enclosed by double quotes ‘"’ in their entirety if they
     contain whitespace or parentheses; within the quotes, only reverse solidus ‘\’ is recognized as an es‐
     cape character.  All string searches are case-insensitive.  When the description indicates that the
     “envelope” representation of an address field is used, this means that the search string is checked
     against both a list constructed as

           '("name" "source" "local-part" "domain-part")'

     for each address, and the addresses without real names from the respective header field.  These search
     expressions can be nested using parentheses, see below for examples.

     (criterion)
               All messages that satisfy the given criterion.
     (criterion1 criterion2 ... criterionN)
               All messages that satisfy all of the given criteria.
     (or criterion1 criterion2)
               All messages that satisfy either criterion1 or criterion2, or both.  To connect more than
               two criteria using ‘or’ specifications have to be nested using additional parentheses, as
               with ‘(or a (or b c))’, since ‘(or a b c)’ really means ‘((a or b) and c)’.  For a simple
               ‘or’ operation of independent criteria on the lowest nesting level, it is possible to
               achieve similar effects by using three separate criteria, as with ‘(a) (b) (c)’.
     (not criterion)
               All messages that do not satisfy criterion.
     (bcc "string")
               All messages that contain string in the envelope representation of the ‘Bcc:’ field.
     (cc "string")
               All messages that contain string in the envelope representation of the ‘Cc:’ field.
     (from "string")
               All messages that contain string in the envelope representation of the ‘From:’ field.
     (subject "string")
               All messages that contain string in the ‘Subject:’ field.
     (to "string")
               All messages that contain string in the envelope representation of the ‘To:’ field.
     (header name "string")
               All messages that contain string in the specified ‘Name:’ field.
     (body "string")
               All messages that contain string in their body.
     (text "string")
               All messages that contain string in their header or body.
     (larger size)
               All messages that are larger than size (in bytes).
     (smaller size)
               All messages that are smaller than size (in bytes).
     (before date)
               All messages that were received before date, which must be in the form ‘d[d]-mon-yyyy’,
               where ‘d’ denotes the day of the month as one or two digits, ‘mon’ is the name of the month
               – one of ‘Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec’, and ‘yyyy’ is the year as four
               digits, e.g., ‘28-Dec-2012’.
     (on date)
               All messages that were received on the specified date.
     (since date)
               All messages that were received since the specified date.
     (sentbefore date)
               All messages that were sent on the specified date.
     (senton date)
               All messages that were sent on the specified date.
     (sentsince date)
               All messages that were sent since the specified date.
     ()        The same criterion as for the previous search.  This specification cannot be used as part of
               another criterion.  If the previous command line contained more than one independent crite‐
               rion then the last of those criteria is used.

   On terminal control and line editor
     [Option] Terminal control will be realized through one of the standard UNIX libraries, either the
     Termcap Access Library (libtermcap, -ltermcap), or, alternatively, the Terminal Information Library
     (libterminfo, -lterminfo), both of which will be initialized to work with the environment variable
     TERM.  Terminal control will enhance or enable interactive usage aspects, e.g., Coloured display, and
     extend behaviour of the Mailx-Line-Editor (MLE), which may learn the byte-sequences of keys like the
     cursor- and function-keys.

     The internal variable termcap can be used to overwrite settings or to learn (correct(ed)) keycodes.
     Actual library interaction can be disabled completely by setting termcap-disable; termcap will be
     queried regardless, which is true even if the [Option]al library support has not been enabled at con‐
     figuration time as long as some other [Option] which (may) query terminal control sequences has been
     enabled.  S-nail can be told to enter an alternative exclusive screen, the so-called ca-mode, by set‐
     ting termcap-ca-mode; this requires sufficient terminal support, and the used PAGER may also need spe‐
     cial configuration, dependent on the value of crt.

     [Option] The built-in Mailx-Line-Editor (MLE) should work in all environments which comply to the ISO
     C standard ISO/IEC 9899/AMD1:1995 (“ISO C90, Amendment 1”), and will support wide glyphs if possible
     (the necessary functionality had been removed from ISO C, but was included in X/Open Portability Guide
     Issue 4 (“XPG4”)).  Usage of a line editor in interactive mode can be prevented by setting
     line-editor-disable.  Especially if the [Option]al terminal control support is missing setting entries
     in the internal variable termcap will help shall the MLE misbehave, see there for more.  The MLE can
     support a little bit of colour.

     [Option] If the history feature is available then input from line editor prompts will be saved in a
     history list that can be searched in and be expanded from.  Such saving can be prevented by prefixing
     input with any amount of whitespace.  Aspects of history, like allowed content and maximum size, as
     well as whether history shall be saved persistently, can be configured with the internal variables
     history-file, history-gabby, history-gabby-persist and history-size.

     The MLE supports a set of editing and control commands.  By default (as) many (as possible) of these
     will be assigned to a set of single-letter control codes, which should work on any terminal (and can
     be generated by holding the “control” key while pressing the key of desire, e.g., ‘control-D’).  If
     the [Option]al bind command is available then the MLE commands can also be accessed freely by assign‐
     ing the command name, which is shown in parenthesis in the list below, to any desired key-sequence,
     and the MLE will instead and also use bind to establish its built-in key bindings (more of them if the
     [Option]al terminal control is available), an action which can then be suppressed completely by set‐
     ting line-editor-no-defaults.  Shell-style argument quoting notation is used in the following; combi‐
     nations not mentioned either cause job control signals or do not generate a (unique) keycode:

     ‘\cA’     Go to the start of the line (mle-go-home).
     ‘\cB’     Move the cursor backward one character (mle-go-bwd).
     ‘\cD’     Forward delete the character under the cursor; quits S-nail if used on the empty line unless
               the internal variable ignoreeof is set (mle-del-fwd).
     ‘\cE’     Go to the end of the line (mle-go-end).
     ‘\cF’     Move the cursor forward one character (mle-go-fwd).
     ‘\cG’     Cancel current operation, full reset.  If there is an active history search or tabulator ex‐
               pansion then this command will first reset that, reverting to the former line content; thus
               a second reset is needed for a full reset in this case (mle-reset).
     ‘\cH’     Backspace: backward delete one character (mle-del-bwd).
     ‘\cI’     [Only new quoting rules] Horizontal tabulator: try to expand the word before the cursor,
               supporting the usual Filename transformations (mle-complete; this is affected by
               mle-quote-rndtrip).
     ‘\cJ’     Newline: commit the current line (mle-commit).
     ‘\cK’     Cut all characters from the cursor to the end of the line (mle-snarf-end).
     ‘\cL’     Repaint the line (mle-repaint).
     ‘\cN’     [Option] Go to the next history entry (mle-hist-fwd).
     ‘\cO’     ([Option]ally context-dependent) Invokes the command dt.
     ‘\cP’     [Option] Go to the previous history entry (mle-hist-bwd).
     ‘\cQ’     Toggle roundtrip mode shell quotes, where produced, on and off (mle-quote-rndtrip).  This
               setting is temporary, and will be forgotten once the command line is committed; also see
               shcodec.
     ‘\cR’     [Option] Complete the current line from (the remaining) older history entries
               (mle-hist-srch-bwd).
     ‘\cS’     [Option] Complete the current line from (the remaining) newer history entries
               (mle-hist-srch-fwd).
     ‘\cT’     Paste the snarf buffer (mle-paste).
     ‘\cU’     The same as ‘\cA’ followed by ‘\cK’ (mle-snarf-line).
     ‘\cV’     Prompts for a Unicode character (hexadecimal number without prefix, see vexpr) to be in‐
               serted (mle-prompt-char).  Note this command needs to be assigned to a single-letter control
               code in order to become recognized and executed during input of a key-sequence (only three
               single-letter control codes can be used for that shortcut purpose); this control code is
               then special-treated and thus cannot be part of any other sequence (because it will trigger
               the mle-prompt-char function immediately).
     ‘\cW’     Cut the characters from the one preceding the cursor to the preceding word boundary
               (mle-snarf-word-bwd).
     ‘\cX’     Move the cursor forward one word boundary (mle-go-word-fwd).
     ‘\cY’     Move the cursor backward one word boundary (mle-go-word-bwd).
     ‘\c[’     Escape: reset a possibly used multibyte character input state machine and [Option]ally a
               lingering, incomplete key binding (mle-cancel).  This command needs to be assigned to a sin‐
               gle-letter control code in order to become recognized and executed during input of a key-se‐
               quence (only three single-letter control codes can be used for that shortcut purpose).  This
               control code may also be part of a multi-byte sequence, but if a sequence is active and the
               very control code is currently also an expected input, then the active sequence takes prece‐
               dence and will consume the control code.
     ‘\c\’     ([Option]ally context-dependent) Invokes the command ‘z+’.
     ‘\c]’     ([Option]ally context-dependent) Invokes the command ‘z$’.
     ‘\c^’     ([Option]ally context-dependent) Invokes the command ‘z0’.
     ‘\c_’     Cut the characters from the one after the cursor to the succeeding word boundary
               (mle-snarf-word-fwd).
     ‘\c?’     Backspace: mle-del-bwd.
     –         Move the cursor forward one screen width (mle-go-screen-fwd).
     –         Move the cursor backward one screen width (mle-go-screen-bwd).
     –         [Option] Move the cursor home and clear the screen (mle-clear-screen).
     –         mle-fullreset: different to mle-reset this will immediately reset a possibly active search
               etc.
     –         mle-bell: ring the audible bell.

   Coloured display
     [Option] S-nail can be configured to support a coloured display and font attributes by emitting ANSI
     a.k.a. ISO 6429 SGR (select graphic rendition) escape sequences.  Usage of colours and font attributes
     solely depends upon the capability of the detected terminal type that is defined by the environment
     variable TERM and which can be fine-tuned by the user via the internal variable termcap.

     On top of what S-nail knows about the terminal the boolean variable colour-pager defines whether the
     actually applicable colour and font attribute sequences should also be generated when output is going
     to be paged through the external program defined by the environment variable PAGER (also see crt).
     This is not enabled by default because different pager programs need different command line switches
     or other configuration in order to support those sequences.  S-nail however knows about some widely
     used pagers and in a clean environment it is often enough to simply set colour-pager; please refer to
     that variable for more on this topic.

     Colours and font attributes can be managed with the multiplexer command colour, and uncolour can be
     used to remove mappings of a given colour type.  If the variable colour-disable is set then any active
     usage of colour and font attribute sequences is suppressed without affecting possibly established
     colour mappings.  Since colours are only available in interactive mode, it may make sense to condi‐
     tionalize the colour setup by encapsulating it with if:

           if terminal && [ "$features" =% +colour ]
             colour iso view-msginfo ft=bold,fg=green
             colour iso view-header ft=bold,fg=red from,subject
             colour iso view-header fg=red

             uncolour iso view-header from,subject
             colour iso view-header ft=bold,fg=magenta,bg=cyan
             colour 256 view-header ft=bold,fg=208,bg=230 "subject,from"
             colour mono view-header ft=bold
             colour mono view-header ft=bold,ft=reverse subject,from
           endif

   Handling spam
     [Option] S-nail can make use of several spam interfaces for the purpose of identification of, and, in
     general, dealing with spam messages.  A precondition of most commands in order to function is that the
     spam-interface variable is set to one of the supported interfaces.  Specifying messages that have been
     identified as spam is possible via their (volatile) ‘is-spam’ state by using the ‘:s’ and ‘:S’ speci‐
     fications, and their attrlist entries will be used when displaying the headline in the summary of
     headers.

     •   spamrate rates the given messages and sets their ‘is-spam’ flag accordingly.  If the spam inter‐
         face offers spam scores these can be shown in headline by using the format ‘%$’.

     •   spamham, spamspam and spamforget will interact with the Bayesian filter of the chosen interface
         and learn the given messages as “ham” or “spam”, respectively; the last command can be used to
         cause “unlearning” of messages; it adheres to their current ‘is-spam’ state and thus reverts pre‐
         vious teachings.

     •   spamclear and spamset will simply set and clear, respectively, the mentioned volatile ‘is-spam’
         message flag, without any interface interaction.

     The spamassassin(1) based spam-interface ‘spamc’ requires a running instance of the spamd(1) server in
     order to function, started with the option --allow-tell shall Bayesian filter learning be possible.

           $ spamd -i localhost:2142 -i /tmp/.spamsock -d [-L] [-l]
           $ spamd --listen=localhost:2142 --listen=/tmp/.spamsock \
               --daemonize [--local] [--allow-tell]

     Thereafter S-nail can make use of these interfaces:

           $ s-nail -Sspam-interface=spamc -Sspam-maxsize=500000 \
               -Sspamc-command=/usr/local/bin/spamc \
               -Sspamc-arguments="-U /tmp/.spamsock" -Sspamc-user=
           or
           $ s-nail -Sspam-interface=spamc -Sspam-maxsize=500000 \
               -Sspamc-command=/usr/local/bin/spamc \
               -Sspamc-arguments="-d localhost -p 2142" -Sspamc-user=

     Using the generic filter approach allows usage of programs like bogofilter(1).  Here is an example,
     requiring it to be accessible via PATH:

           $ s-nail -Sspam-interface=filter -Sspam-maxsize=500000 \
               -Sspamfilter-ham="bogofilter -n" \
               -Sspamfilter-noham="bogofilter -N" \
               -Sspamfilter-nospam="bogofilter -S" \
               -Sspamfilter-rate="bogofilter -TTu 2>/dev/null" \
               -Sspamfilter-spam="bogofilter -s" \
               -Sspamfilter-rate-scanscore="1;^(.+)$"

     Because messages must exist on local storage in order to be scored (or used for Bayesian filter train‐
     ing), it is possibly a good idea to perform the local spam check last.  Spam can be checked automati‐
     cally when opening specific folders by setting a specialized form of the internal variable
     folder-hook.

           define spamdelhook {
             # Server side DCC
             spamset (header x-dcc-brand-metrics "bulk")
             # Server-side spamassassin(1)
             spamset (header x-spam-flag "YES")
             del :s # TODO we HAVE to be able to do `spamrate :u ! :sS'
             move :S +maybe-spam
             spamrate :u
             del :s
             move :S +maybe-spam
           }
           set folder-hook-SOMEFOLDER=spamdelhook

     See also the documentation for the variables spam-interface, spam-maxsize, spamc-command,
     spamc-arguments, spamc-user, spamfilter-ham, spamfilter-noham, spamfilter-nospam, spamfilter-rate and
     spamfilter-rate-scanscore.

COMMANDS
     S-nail reads input in lines.  An unquoted reverse solidus ‘\’ at the end of a command line “escapes”
     the newline character: it is discarded and the next line of input is used as a follow-up line, with
     all leading whitespace removed; once an entire line is completed, the whitespace characters space,
     tabulator, newline as well as those defined by the variable ifs are removed from the beginning and
     end.  Placing any whitespace characters at the beginning of a line will prevent a possible addition of
     the command line to the [Option]al history.

     The beginning of such input lines is then scanned for the name of a known command: command names may
     be abbreviated, in which case the first command that matches the given prefix will be used.  Command
     modifiers may prefix a command in order to modify its behaviour.  A name may also be a commandalias,
     which will become expanded until no more expansion is possible.  Once the command that shall be exe‐
     cuted is known, the remains of the input line will be interpreted according to command-specific rules,
     documented in the following.

     This behaviour is different to the sh(1)ell, which is a programming language with syntactic elements
     of clearly defined semantics, and therefore capable to sequentially expand and evaluate individual el‐
     ements of a line.  S-nail will never be able to handle ‘? set one=value two=$one’ in a single state‐
     ment, because the variable assignment is performed by the command (set), not the language.

     The command list can be used to show the list of all commands, either alphabetically sorted or in pre‐
     fix search order (these do not match, also because the POSIX standard prescribes a set of abbrevia‐
     tions).  [Option]ally the command help (or ?), when given an argument, will show a documentation
     string for the command matching the expanded argument, as in ‘?t’, which should be a shorthand of
     ‘?type’; with these documentation strings both commands support a more verbose listing mode which in‐
     cludes the argument type of the command and other information which applies; a handy suggestion might
     thus be:

           ? define __xv {
             # Before v15: need to enable sh(1)ell-style on _entire_ line!
             localopts yes;wysh set verbose;ignerr eval "${@}";return ${?}
           }
           ? commandalias xv '\call __xv'
           ? xv help set

   Command modifiers
     Commands may be prefixed by one or multiple command modifiers.  Some command modifiers can be used
     with a restricted set of commands only, the verbose version of list will ([Option]ally) show which
     modifiers apply.

     •   The modifier reverse solidus \, to be placed first, prevents commandalias expansions on the re‐
         mains of the line, e.g., ‘\echo’ will always evaluate the command echo, even if an (command)alias
         of the same name exists.  commandalias content may itself contain further command modifiers, in‐
         cluding an initial reverse solidus to prevent further expansions.

     •   The modifier ignerr indicates that any error generated by the following command should be ignored
         by the state machine and not cause a program exit with enabled errexit or for the standardized
         exit cases in posix mode.  ?, one of the INTERNAL VARIABLES, will be set to the real exit status
         of the command regardless.

     •   local will alter the called command to apply changes only temporarily, local to block-scope, and
         can thus only be used inside of a defined macro or an account definition.  Specifying it implies
         the modifier wysh.  Block-scope settings will not be inherited by macros deeper in the call chain,
         and will be garbage collected once the current block is left.  To record and unroll changes in the
         global scope use the command localopts.

     •   scope does yet not implement any functionality.

     •   u does yet not implement any functionality.

     •   Some commands support the vput modifier: if used, they expect the name of a variable, which can
         itself be a variable, i.e., shell expansion is applied, as their first argument, and will place
         their computation result in it instead of the default location (it is usually written to standard
         output).

         The given name will be tested for being a valid sh(1) variable name, and may therefore only con‐
         sist of upper- and lowercase characters, digits, and the underscore; the hyphen-minus may be used
         as a non-portable extension; digits may not be used as first, hyphen-minus may not be used as last
         characters.  In addition the name may either not be one of the known INTERNAL VARIABLES, or must
         otherwise refer to a writable (non-boolean) value variable.  The actual put operation may fail
         nonetheless, e.g., if the variable expects a number argument only a number will be accepted.  Any
         error during these operations causes the command as such to fail, and the error number ! will be
         set to ^ERR-NOTSUP, the exit status ? should be set to ‘-1’, but some commands deviate from the
         latter, which is documented.

     •   Last, but not least, the modifier wysh can be used for some old and established commands to choose
         the new Shell-style argument quoting rules over the traditional Old-style argument quoting.

   Old-style argument quoting
     [v15 behaviour may differ] This section documents the old, traditional style of quoting non-message-
     list arguments to commands which expect this type of arguments: whereas still used by the majority of
     such commands, the new Shell-style argument quoting may be available even for those via wysh, one of
     the Command modifiers.  Nonetheless care must be taken, because only new commands have been designed
     with all the capabilities of the new quoting rules in mind, which can, e.g., generate control charac‐
     ters.

           •   An argument can be enclosed between paired double-quotes ‘"argument"’ or single-quotes
               ‘'argument'’; any whitespace, shell word expansion, or reverse solidus characters (except as
               described next) within the quotes are treated literally as part of the argument.  A double-
               quote will be treated literally within single-quotes and vice versa.  Inside such a quoted
               string the actually used quote character can be used nonetheless by escaping it with a re‐
               verse solidus ‘\’, as in ‘"y\"ou"’.

           •   An argument that is not enclosed in quotes, as above, can usually still contain space char‐
               acters if those spaces are reverse solidus escaped, as in ‘you\ are’.

           •   A reverse solidus outside of the enclosing quotes is discarded and the following character
               is treated literally as part of the argument.

   Shell-style argument quoting
     sh(1)ell-style, and therefore POSIX standardized, argument parsing and quoting rules are used by most
     commands.  [v15 behaviour may differ] Most new commands only support these new rules and are flagged
     [Only new quoting rules], some elder ones can use them with the command modifier wysh; in the future
     only this type of argument quoting will remain.

     A command line is parsed from left to right and an input token is completed whenever an unquoted, oth‐
     erwise ignored, metacharacter is seen.  Metacharacters are vertical bar |, ampersand &, semicolon ;,
     as well as all characters from the variable ifs, and / or space, tabulator, newline.  The additional
     metacharacters left and right parenthesis (, ) and less-than and greater-than signs <, > that the
     sh(1) supports are not used, and are treated as ordinary characters: for one these characters are a
     vivid part of email addresses, and it seems highly unlikely that their function will become meaningful
     to S-nail.

           Compatibility note: [v15 behaviour may differ] Please note that even many new-style commands  do
           not  yet  honour ifs to parse their arguments: whereas the sh(1)ell is a language with syntactic
           elements of clearly defined semantics, S-nail parses entire input lines and decides  on  a  per-
           command  base  what  to  do with the rest of the line.  This also means that whenever an unknown
           command is seen all that S-nail can do is cancellation of the processing of the remains  of  the
           line.

           It  also often depends on an actual subcommand of a multiplexer command how the rest of the line
           should be treated, and until v15 we are not capable to perform this  deep  inspection  of  argu‐
           ments.  Nonetheless, at least the following commands which work with positional parameters fully
           support ifs for an almost shell-compatible field splitting: call, call_if, read, vpospar, xcall.

     Any unquoted number sign ‘#’ at the beginning of a new token starts a comment that extends to the end
     of the line, and therefore ends argument processing.  An unquoted dollar sign ‘$’ will cause variable
     expansion of the given name, which must be a valid sh(1)ell-style variable name (see vput): INTERNAL
     VARIABLES as well as ENVIRONMENT (shell) variables can be accessed through this mechanism, brace en‐
     closing the name is supported (i.e., to subdivide a token).

     Whereas the metacharacters space, tabulator, newline only complete an input token, vertical bar |, am‐
     persand & and semicolon ; also act as control operators and perform control functions.  For now sup‐
     ported is semicolon ;, which terminates a single command, therefore sequencing the command line and
     making the remainder of the line a subject to reevaluation.  With sequencing, multiple command argu‐
     ment types and quoting rules may therefore apply to a single line, which can become problematic before
     v15: e.g., the first of the following will cause surprising results.

           ? echo one; set verbose; echo verbose=$verbose.
           ? echo one; wysh set verbose; echo verbose=$verbose.

     Quoting is a mechanism that will remove the special meaning of metacharacters and reserved words, and
     will prevent expansion.  There are four quoting mechanisms: the escape character, single-quotes, dou‐
     ble-quotes and dollar-single-quotes:

           •   The literal value of any character can be preserved by preceding it with the escape charac‐
               ter reverse solidus ‘\’.

           •   Arguments which are enclosed in ‘'single-quotes'’ retain their literal value.  A single-
               quote cannot occur within single-quotes.

           •   The literal value of all characters enclosed in ‘"double-quotes"’ is retained, with the ex‐
               ception of dollar sign ‘$’, which will cause variable expansion, as above, backquote (grave
               accent) ‘`’, (which not yet means anything special), reverse solidus ‘\’, which will escape
               any of the characters dollar sign ‘$’ (to prevent variable expansion), backquote (grave ac‐
               cent) ‘`’, double-quote ‘"’ (to prevent ending the quote) and reverse solidus ‘\’ (to pre‐
               vent escaping, i.e., to embed a reverse solidus character as-is), but has no special meaning
               otherwise.

           •   Arguments enclosed in ‘$'dollar-single-quotes'’ extend normal single quotes in that reverse
               solidus escape sequences are expanded as follows:

               ‘\a’    bell control character (ASCII and ISO-10646 BEL).
               ‘\b’    backspace control character (ASCII and ISO-10646 BS).
               ‘\E’    escape control character (ASCII and ISO-10646 ESC).
               ‘\e’    the same.
               ‘\f’    form feed control character (ASCII and ISO-10646 FF).
               ‘\n’    line feed control character (ASCII and ISO-10646 LF).
               ‘\r’    carriage return control character (ASCII and ISO-10646 CR).
               ‘\t’    horizontal tabulator control character (ASCII and ISO-10646 HT).
               ‘\v’    vertical tabulator control character (ASCII and ISO-10646 VT).
               ‘\\’    emits a reverse solidus character.
               ‘\'’    single quote.
               ‘\"’    double quote (escaping is optional).
               ‘\NNN’  eight-bit byte with the octal value ‘NNN’ (one to three octal digits), optionally
                       prefixed by an additional ‘0’.  A 0 byte will suppress further output for the quoted
                       argument.
               ‘\xHH’  eight-bit byte with the hexadecimal value ‘HH’ (one or two hexadecimal characters,
                       no prefix, see vexpr).  A 0 byte will suppress further output for the quoted argu‐
                       ment.
               ‘\UHHHHHHHH’
                       the Unicode / ISO-10646 character with the hexadecimal codepoint value ‘HHHHHHHH’
                       (one to eight hexadecimal characters) — note that Unicode defines the maximum code‐
                       point ever to be supported as ‘0x10FFFF’ (in planes of ‘0xFFFF’ characters each).
                       This escape is only supported in locales that support Unicode (see Character sets),
                       in other cases the sequence will remain unexpanded unless the given code point is
                       ASCII compatible or (if the [Option]al character set conversion is available) can be
                       represented in the current locale.  The character NUL will suppress further output
                       for the quoted argument.
               ‘\uHHHH’
                       Identical to ‘\UHHHHHHHH’ except it takes only one to four hexadecimal characters.
               ‘\cX’   Emits the non-printable (ASCII and compatible) C0 control codes 0 (NUL) to 31 (US),
                       and 127 (DEL).  Printable representations of ASCII control codes can be created by
                       mapping them to a different, visible part of the ASCII character set.  Adding the
                       number 64 achieves this for the codes 0 to 31, e.g., 7 (BEL): ‘7 + 64 = 71 = G’.
                       The real operation is a bitwise logical XOR with 64 (bit 7 set, see vexpr), thus
                       also covering code 127 (DEL), which is mapped to 63 (question mark): ‘? vexpr ^ 127
                       64’.

                       Whereas historically circumflex notation has often been used for visualization pur‐
                       poses of control codes, e.g., ‘^G’, the reverse solidus notation has been standard‐
                       ized: ‘\cG’.  Some control codes also have standardized (ISO-10646, ISO C) aliases,
                       as shown above (e.g., ‘\a’, ‘\n’, ‘\t’): whenever such an alias exists it will be
                       used for display purposes.  The control code NUL (‘\c@’, a non-standard extension)
                       will suppress further output for the remains of the token (which may extend beyond
                       the current quote), or, depending on the context, the remains of all arguments for
                       the current command.
               ‘\$NAME’
                       Non-standard extension: expand the given variable name, as above.  Brace enclosing
                       the name is supported.
               ‘\`{command}’
                       Not yet supported, just to raise awareness: Non-standard extension.

     Caveats:

           ? echo 'Quotes '${HOME}' and 'tokens" differ!"# no comment
           ? echo Quotes ${HOME} and tokens differ! # comment
           ? echo Don"'"t you worry$'\x21' The sun shines on us. $'\u263A'

   Message list arguments
     Many commands operate on message list specifications, as documented in Specifying messages.  The argu‐
     ment input is first split into individual tokens via Shell-style argument quoting, which are then in‐
     terpreted as the mentioned specifications.  If no explicit message list has been specified, many com‐
     mands will search for and use the next message forward that satisfies the commands' requirements, and
     if there are no messages forward of the current message, the search proceeds backwards; if there are
     no good messages at all to be found, an error message is shown and the command is aborted.  The
     verbose output of the command list will indicate whether a command searches for a default message, or
     not.

   Raw data arguments for codec commands
     A special set of commands, which all have the string “codec” in their name, e.g., addrcodec, shcodec,
     urlcodec, take raw string data as input, which means that the content of the command input line is
     passed completely unexpanded and otherwise unchanged: like this the effect of the actual codec is vis‐
     ible without any noise of possible shell quoting rules etc., i.e., the user can input one-to-one the
     desired or questionable data.  To gain a level of expansion, the entire command line can be evaluated
     first, e.g.,

           ? vput shcodec res encode /usr/Schönes Wetter/heute.txt
           ? echo $res
           $'/usr/Sch\u00F6nes Wetter/heute.txt'
           ? shcodec d $res
           $'/usr/Sch\u00F6nes Wetter/heute.txt'
           ? eval shcodec d $res
           /usr/Schönes Wetter/heute.txt

   Filename transformations
     Filenames, where expected, and unless documented otherwise, are subsequently subject to the following
     filename transformations, in sequence:

           •   If the given name is a registered shortcut, it will be replaced with the expanded shortcut.

           •   The filename is matched against the following patterns or strings:

               #      (Number sign) is expanded to the previous file.
               %      (Percent sign) is replaced by the invoking user's primary system mailbox, which ei‐
                      ther is the (itself expandable) inbox if that is set, the standardized absolute path‐
                      name indicated by MAIL if that is set, or a built-in compile-time default otherwise.
               %user  Expands to the primary system mailbox of user (and never the value of inbox, regard‐
                      less of its actual setting).
               &      (Ampersand) is replaced with the invoking user's secondary mailbox, the MBOX.
               +file  Refers to a file in the folder directory (if that variable is set).
               %:filespec Expands to the same value as filespec, but has special meaning when used with,
                      e.g., the command file: the file will be treated as a primary system mailbox by,
                      e.g., the mbox and save commands, meaning that messages that have been read in the
                      current session will be moved to the MBOX mailbox instead of simply being flagged as
                      read.

           •   Meta expansions may be applied to the resulting filename, as allowed by the operation and
               applicable to the resulting access protocol (also see On URL syntax and credential lookup).
               For the file-protocol, a leading tilde ‘~’ character will be replaced by the expansion of
               HOME, except when followed by a valid user name, in which case the home directory of the
               given user is used instead.

               A shell expansion as if specified in double-quotes (see Shell-style argument quoting) may be
               applied, so that any occurrence of ‘$VARIABLE’ (or ‘${VARIABLE}’) will be replaced by the
               expansion of the variable, if possible; INTERNAL VARIABLES as well as ENVIRONMENT (shell)
               variables can be accessed through this mechanism.

               Shell pathname wildcard pattern expansions (glob(7)) may be applied as documented.  If the
               fully expanded filename results in multiple pathnames and the command is expecting only one
               file, an error results.

               In interactive context, in order to allow simple value acceptance (via “ENTER”), arguments
               will usually be displayed in a properly quoted form, e.g., a file ‘diet\ is \curd.txt’ may
               be displayed as ‘'diet\ is \curd.txt'’.

   Commands
     The following commands are available:

     !         Executes the SHELL command which follows, replacing unescaped exclamation marks with the
               previously executed command if the internal variable bang is set.  This command supports
               vput as documented in Command modifiers, and manages the error number !.  A 0 or positive
               exit status ? reflects the exit status of the command, negative ones that an error happened
               before the command was executed, or that the program did not exit cleanly, but, e.g., due to
               a signal: the error number is ^ERR-CHILD, then.

               In conjunction with the vput modifier the following special cases exist: a negative exit
               status occurs if the collected data could not be stored in the given variable, which is a
               ^ERR-NOTSUP error that should otherwise not occur.  ^ERR-CANCELED indicates that no tempo‐
               rary file could be created to collect the command output at first glance.  In case of catch‐
               able out-of-memory situations ^ERR-NOMEM will occur and S-nail will try to store the empty
               string, just like with all other detected error conditions.

     #         The comment-command causes the entire line to be ignored.  Note: this really is a normal
               command which' purpose is to discard its arguments, not a “comment-start” indicating special
               character, which means that, e.g., trailing comments on a line are not possible (except for
               commands which use Shell-style argument quoting).

     +         Goes to the next message in sequence and types it (like “ENTER”).

     -         Display the preceding message, or the n'th previous message if given a numeric argument n.

     =         Shows the message number of the current message (the “dot”) when used without arguments,
               that of the given list otherwise.  Output numbers will be separated from each other with the
               first character of ifs, and followed by the first character of if-ws, if that is not empty
               and not identical to the first.  If that results in no separation at all a space character
               is used.  This command supports vput (see Command modifiers), and manages the error number
               !.

     ?         [Option] Show a brief summary of commands.  [Option] Given an argument a synopsis for the
               command in question is shown instead; commands can be abbreviated in general and this com‐
               mand can be used to see the full expansion of an abbreviation including the synopsis, try,
               e.g., ‘?h’, ‘?hel’ and ‘?help’ and see how the output changes.  This mode also supports a
               more verbose output, which will provide the information documented for list.

     |         A synonym for the pipe command.

     account, unaccount
               (ac, una) Creates, selects or lists (an) account(s).  Accounts are special incarnations of
               defined macros and group commands and variable settings which together usually arrange the
               environment for the purpose of creating an email account.  Different to normal macros set‐
               tings which are covered by localopts – here by default enabled! – will not be reverted be‐
               fore the account is changed again.  The special account ‘null’ (case-insensitive) always ex‐
               ists, and all but it can be deleted by the latter command, and in one operation with the
               special name ‘*’.  Also for all but it a possibly set on-account-cleanup hook is called once
               they are left.

               Without arguments a listing of all defined accounts is shown.  With one argument the given
               account is activated: the system inbox of that account will be activated (as via file), a
               possibly installed folder-hook will be run, and the internal variable account will be up‐
               dated.  The two argument form is identical to defining a macro as via define:

                     account myisp {
                       set folder=~/mail inbox=+syste.mbox record=+sent.mbox
                       set from='(My Name) myname@myisp.example'
                       set mta=smtp://mylogin@smtp.myisp.example
                     }

     addrcodec
               Perform email address codec transformations on raw-data argument, rather according to email
               standards (RFC 5322; [v15 behaviour may differ] will furtherly improve).  Supports vput (see
               Command modifiers), and manages the error number !.  The first argument must be either
               [+[+[+]]]e[ncode], d[ecode], s[kin] or skinl[ist] and specifies the operation to perform on
               the rest of the line.

               Decoding will show how a standard-compliant MUA will display the given argument, which
               should be an email address.  Please be aware that most MUAs have difficulties with the ad‐
               dress standards, and vary wildly when (comments) in parenthesis, “double-quoted” strings, or
               quoted-pairs, as below, become involved.  [v15 behaviour may differ] S-nail currently does
               not perform decoding when displaying addresses.

               Skinning is identical to decoding but only outputs the plain address, without any string,
               comment etc. components.  Another difference is that it may fail with the error number ! set
               to ^ERR-INVAL if decoding fails to find a(n) (valid) email address, in which case the unmod‐
               ified input will be output again.

               skinlist first performs a skin operation, and thereafter checks a valid address for whether
               it is a registered mailing list (see mlist and mlsubscribe), eventually reporting that state
               in the error number ! as ^ERR-EXIST.  (This state could later become overwritten by an I/O
               error, though.)

               Encoding supports four different modes, lesser automated versions can be chosen by prefixing
               one, two or three plus signs: the standard imposes a special meaning on some characters,
               which thus have to be transformed to so-called quoted-pairs by pairing them with a reverse
               solidus ‘\’ in order to remove the special meaning; this might change interpretation of the
               entire argument from what has been desired, however!  Specify one plus sign to remark that
               parenthesis shall be left alone, two for not turning double quotation marks into quoted-
               pairs, and three for also leaving any user-specified reverse solidus alone.  The result will
               always be valid, if a successful exit status is reported ([v15 behaviour may differ] the
               current parser fails this assertion for some constructs).  [v15 behaviour may differ] Ad‐
               dresses need to be specified in between angle brackets ‘<’, ‘>’ if the construct becomes
               more difficult, otherwise the current parser will fail; it is not smart enough to guess
               right.

                     ? addrc enc "Hey, you",<diet@exam.ple>\ out\ there
                     "\"Hey, you\", \\ out\\ there" <diet@exam.ple>
                     ? addrc d "\"Hey, you\", \\ out\\ there" <diet@exam.ple>
                     "Hey, you", \ out\ there <diet@exam.ple>
                     ? addrc s "\"Hey, you\", \\ out\\ there" <diet@exam.ple>
                     diet@exam.ple

     alias, unalias
               (a, una) Aliases are a method of creating personal distribution lists that map a single
               alias name to none to multiple real receivers; these aliases become expanded after message
               composing is completed.  The latter command removes the given list of aliases, the special
               name ‘*’ will discard all existing aliases.

               The former command shows all currently defined aliases when used without arguments, and with
               one argument the expansion of the given alias.  With more than one argument, creates or ap‐
               pends to the alias name given as the first argument the remaining arguments.  Alias names
               adhere to the Postfix MTA aliases(5) rules and are thus restricted to alphabetic characters,
               digits, the underscore, hyphen-minus, the number sign, colon and commercial at, a dollar
               sign is allowed but in the first position; As extensions the exclamation mark ‘!’, period
               ‘.’ as well as “any haracter that has the high bit set” may be used:
               ‘[[:alnum:]_#:@-][[:alnum:]_#:@$;.-]*’.

               [v15 behaviour may differ] Unfortunately the colon is currently not supported, as it inter‐
               feres with normal address parsing rules.  [v15 behaviour may differ] Such high bit charac‐
               ters will likely cause warnings at the moment for the same reasons why colon is unsupported;
               also, in the future locale dependent character set validity checks will be performed.

     alternates, unalternates
               [Only new quoting rules] (alt) Manage a list of alternate addresses or names of the active
               user, members of which will be removed from recipient lists (except one).  There is a set of
               implicit alternates which is formed of the values of LOGNAME, from, sender and reply-to.
               from will not be used if sender is set.  The latter command removes the given list of alter‐
               nates, the special name ‘*’ will discard all existing alternate names.

               The former command manages the error number !.  It shows the current set of alternates when
               used without arguments; in this mode only it also supports vput (see Command modifiers).
               Otherwise the given arguments (after being checked for validity) are appended to the list of
               alternate names; in posix mode they replace that list instead.

     answered, unanswered
               Take a message lists and mark each message as (not) having been answered.  Messages will be
               marked answered when being replyd to automatically if the markanswered variable is set.  See
               the section Message states.

     bind, unbind
               [Option][Only new quoting rules] The bind command extends the MLE (see On terminal control
               and line editor) with freely configurable key bindings.  The latter command removes from the
               given context the given key binding, both of which may be specified as a wildcard ‘*’, so
               that, e.g., ‘unbind * *’ will remove all bindings of all contexts.  Due to initialization
               order unbinding will not work for built-in key bindings upon program startup, however:
               please use line-editor-no-defaults for this purpose instead.

               With one argument the former command shows all key bindings for the given context, specify‐
               ing an asterisk ‘*’ will show the bindings of all contexts; a more verbose listing will be
               produced if either of debug or verbose are set.  With two or more arguments a binding is
               (re)established: the first argument is the context to which the binding shall apply, the
               second argument is a comma-separated list of the “keys” which form the binding, and any re‐
               maining arguments form the expansion.  To indicate that a binding shall not be auto-commit‐
               ted, but that the expansion shall instead be furtherly editable by the user, a commercial at
               ‘@’ (that will be removed) can be placed last in the expansion, from which leading and
               trailing whitespace will finally be removed.  Reverse solidus cannot be used as the last
               character of expansion.

               Contexts define when a binding applies, i.e., a binding will not be seen unless the context
               for which it is defined for is currently active.  This is not true for the shared binding
               ‘base’, which is the foundation for all other bindings and as such always applies, its bind‐
               ings, however, only apply secondarily.  The available contexts are the shared ‘base’, the
               ‘default’ context which is used in all not otherwise documented situations, and ‘compose’,
               which applies to compose mode only.

               “Keys” which form the binding are specified as a comma-separated list of byte-sequences,
               where each list entry corresponds to one key(press).  A list entry may, indicated by a lead‐
               ing colon character ‘:’, also refer to the name of a terminal capability; several dozen
               names will be compiled in and may be specified either by their terminfo(5), or, if existing,
               by their termcap(5) name, regardless of the actually used [Option]al terminal control li‐
               brary.  It is possible to use any capability, as long as the name is resolvable by the [Op‐
               tion]al control library or was defined via the internal variable termcap.  Input sequences
               are not case-normalized, so that an exact match is required to update or remove a binding.
               Examples:

                     ? bind base $'\E',d mle-snarf-word-fwd # Esc(ape)
                     ? bind base $'\E',$'\c?' mle-snarf-word-bwd # Esc,Delete
                     ? bind default $'\cA',:khome,w 'echo Editable binding@'
                     ? bind default a,b,c rm -irf / @  # Also editable
                     ? bind default :kf1 File %
                     ? bind compose :kf1 ~v

               Note that the entire comma-separated list is first parsed (over) as a shell-token with
               whitespace as the field separator, before being parsed and expanded for real with comma as
               the field separator, therefore whitespace needs to be properly quoted, see Shell-style
               argument quoting.  Using Unicode reverse solidus escape sequences renders a binding defunc‐
               tional if the locale does not support Unicode (see Character sets), and using terminal capa‐
               bilities does so if no (corresponding) terminal control support is (currently) available.

               The following terminal capability names are built-in and can be used in terminfo(5) or (if
               available) the two-letter termcap(5) notation.  See the respective manual for a list of ca‐
               pabilities.  The program infocmp(1) can be used to show all the capabilities of TERM or the
               given terminal type; using the -x flag will also show supported (non-standard) extensions.

               kbs or kb       Backspace.
               kdch1 or kD     Delete character.
               kDC or *4       — shifted variant.
               kel or kE       Clear to end of line.
               kext or @9      Exit.
               kich1 or kI     Insert character.
               kIC or #3       — shifted variant.
               khome or kh     Home.
               kHOM or #2      — shifted variant.
               kend or @7      End.
               knp or kN       Next page.
               kpp or kP       Previous page.
               kcub1 or kl     Left cursor (with more modifiers: see below).
               kLFT or #4      — shifted variant.
               kcuf1 or kr     Right cursor (ditto).
               kRIT or %i      — shifted variant.
               kcud1 or kd     Down cursor (ditto).
               kDN             — shifted variant (only terminfo).
               kcuu1 or ku     Up cursor (ditto).
               kUP             — shifted variant (only terminfo).
               kf0 or k0       Function key 0.  Add one for each function key up to kf9 and k9, respec‐
                               tively.
               kf10 or k;      Function key 10.
               kf11 or F1      Function key 11.  Add one for each function key up to kf19 and F9, respec‐
                               tively.

               Some terminals support key-modifier combination extensions, e.g., ‘Alt+Shift+xy’.  For exam‐
               ple, the delete key, kdch1: in its shifted variant, the name is mutated to kDC, then a num‐
               ber is appended for the states ‘Alt’ (kDC3), ‘Shift+Alt’ (kDC4), ‘Control’ (kDC5),
               ‘Shift+Control’ (kDC6), ‘Alt+Control’ (kDC7), finally ‘Shift+Alt+Control’ (kDC8).  The same
               for the left cursor key, kcub1: KLFT, KLFT3, KLFT4, KLFT5, KLFT6, KLFT7, KLFT8.

               It is advisable to use an initial escape or other control character (e.g., ‘\cA’) for bind‐
               ings which describe user key combinations (as opposed to purely terminal capability based
               ones), in order to avoid ambiguities whether input belongs to key sequences or not; it also
               reduces search time.  Adjusting bind-timeout may help shall keys and sequences be falsely
               recognized.

     call      [Only new quoting rules] Calls the given macro, which must have been created via define (see
               there for more), otherwise an ^ERR-NOENT error occurs.  Calling macros recursively will at
               some time excess the stack size limit, causing a hard program abortion; if recursively call‐
               ing a macro is the last command of the current macro, consider to use the command xcall,
               which will first release all resources of the current macro before replacing the current
               macro with the called one.

     call_if   Identical to call if the given macro has been created via define, but does not fail nor warn
               if the macro does not exist.

     cd        (ch) Change the working directory to HOME or the given argument.  Synonym for chdir.

     certsave  [Option] Only applicable to S/MIME signed messages.  Takes an optional message list and a
               filename and saves the certificates contained within the message signatures to the named
               file in both human-readable and PEM format.  The certificates can later be used to send en‐
               crypted messages to the respective message senders by setting smime-encrypt-USER@HOST vari‐
               ables.

     charsetalias, uncharsetalias
               [Only new quoting rules] Manage alias mappings for (conversion of) Character sets.  Mappings
               are ineffective if character set conversion is not available (features does not announce
               ‘+iconv’).  Expansion happens recursively, but expansion is not performed for INTERNAL
               VARIABLES, e.g., charset-8bit.

               The latter command deletes all aliases given as arguments, all aliases can be deleted at
               once with the special argument ‘*’.  The former shows the list of all currently defined
               aliases if used without arguments, the expansion of the given alias with one argument.  Oth‐
               erwise all given arguments are treated as pairs of character sets and their desired target
               alias name, creating new or changing already existing aliases, as necessary.

     chdir     (ch) Change the working directory to HOME or the given argument.  Synonym for cd.

     collapse, uncollapse
               Only applicable to ‘thread’ed sort mode.  Takes a message list and makes all replies to
               these messages invisible in header summaries, except for ‘new’ messages and the “dot”.  Also
               when a message with collapsed replies is displayed, all of these are automatically uncol‐
               lapsed.  The latter command undoes collapsing.

     colour, uncolour
               [Option][Only new quoting rules] Manage colour mappings of and for a Coloured display.  The
               type of colour is given as the (case-insensitive) first argument, which must be one of ‘256’
               for 256-colour terminals, ‘8’, ‘ansi’ or ‘iso’ for the standard 8-colour ANSI / ISO 6429
               colour palette and ‘1’ or ‘mono’ for monochrome terminals.  Monochrome terminals cannot deal
               with colours, but only (some) font attributes.

               Without further arguments the list of all currently defined mappings for the given colour
               type is shown (as a special case giving ‘all’ or ‘*’ will show the mappings of all types).
               Otherwise the second argument defines the mappable slot, and the third argument a (comma-
               separated list of) colour and font attribute specification(s), and the optional fourth argu‐
               ment can be used to specify a precondition: if conditioned mappings exist they are tested in
               (creation) order unless a (case-insensitive) match has been found, and the default mapping
               (if any has been established) will only be chosen as a last resort.  The types of precondi‐
               tion available depend on the mappable slot (see Coloured display for some examples), the
               following of which exist:

               Mappings prefixed with ‘mle-’ are used for the [Option]al built-in Mailx-Line-Editor (MLE,
               see On terminal control and line editor) and do not support preconditions.

               mle-position   This mapping is used for the position indicator that is visible when a line
                              cannot be fully displayed on the screen.
               mle-prompt     Used for the prompt.

               Mappings prefixed with ‘sum-’ are used in header summaries, and they all understand the pre‐
               conditions ‘dot’ (the current message) and ‘older’ for elder messages (only honoured in con‐
               junction with datefield-markout-older).

               sum-dotmark    This mapping is used for the “dotmark” that can be created with the ‘%>’ or
                              ‘%<’ formats of the variable headline.
               sum-header     For the complete header summary line except the “dotmark” and the thread
                              structure.
               sum-thread     For the thread structure which can be created with the ‘%i’ format of the
                              variable headline.

               Mappings prefixed with ‘view-’ are used when displaying messages.

               view-from_     This mapping is used for so-called ‘From_’ lines, which are MBOX file format
                              specific header lines.
               view-header    For header lines.  A comma-separated list of headers to which the mapping ap‐
                              plies may be given as a precondition; if the [Option]al regular expression
                              support is available then if any of the “magical” (extended) regular expres‐
                              sion characters is seen the precondition will be evaluated as (an extended)
                              one.
               view-msginfo   For the introductional message info line.
               view-partinfo  For MIME part info lines.

               The following (case-insensitive) colour definitions and font attributes are understood, mul‐
               tiple of which can be specified in a comma-separated list:

               ft=  a font attribute: ‘bold’, ‘reverse’ or ‘underline’.  It is possible (and often applica‐
                    ble) to specify multiple font attributes for a single mapping.

               fg=  foreground colour attribute: ‘black’, ‘blue’, ‘green’, ‘red’, ‘brown’, ‘magenta’,
                    ‘cyan’ or ‘white’.  To specify a 256-colour mode a decimal number colour specification
                    in the range 0 to 255, inclusive, is supported, and interpreted as follows:

                    0 - 7      the standard ISO 6429 colours, as above.
                    8 - 15     high intensity variants of the standard colours.
                    16 - 231   216 colours in tuples of 6.
                    232 - 255  grayscale from black to white in 24 steps.

                          #!/bin/sh -
                          fg() { printf "\033[38;5;${1}m($1)"; }
                          bg() { printf "\033[48;5;${1}m($1)"; }
                          i=0
                          while [ $i -lt 256 ]; do fg $i; i=$(($i + 1)); done
                          printf "\033[0m\n"
                          i=0
                          while [ $i -lt 256 ]; do bg $i; i=$(($i + 1)); done
                          printf "\033[0m\n"

               bg=  background colour attribute (see fg= for possible values).

               The command uncolour will remove for the given colour type (the special type ‘*’ selects
               all) the given mapping; if the optional precondition argument is given only the exact tuple
               of mapping and precondition is removed.  The special name ‘*’ will remove all mappings (no
               precondition allowed), thus ‘uncolour * *’ will remove all established mappings.

     commandalias, uncommandalias
               [Only new quoting rules] Define or list, and remove, respectively, command aliases.  An
               (command)alias can be used everywhere a normal command can be used, but always takes prece‐
               dence: any arguments that are given to the command alias are joined onto the alias expan‐
               sion, and the resulting string forms the command line that is, in effect, executed.  The
               latter command removes all given aliases, the special name ‘*’ will remove all existing
               aliases.  When used without arguments the former shows a list of all currently known
               aliases, with one argument only the expansion of the given one.

               With two or more arguments a command alias is defined or updated: the first argument is the
               name under which the remaining command line should be accessible, the content of which can
               be just about anything.  An alias may itself expand to another alias, but to avoid expansion
               loops further expansion will be prevented if an alias refers to itself or if an expansion
               depth limit is reached.  Explicit expansion prevention is available via reverse solidus \,
               one of the Command modifiers.

                     ? commandalias xx
                     s-nail: `commandalias': no such alias: xx
                     ? commandalias xx echo hello,
                     ? commandalias xx
                     commandalias xx 'echo hello,'
                     ? xx
                     hello,
                     ? xx world
                     hello, world

     Copy      (C) Copy messages to files whose names are derived from the author of the respective message
               and do not mark them as being saved; otherwise identical to Save.

     copy      (c) Copy messages to the named file and do not mark them as being saved; otherwise identical
               to save.

     cwd       Show the name of the current working directory, as reported by getcwd(3).  Supports vput
               (see Command modifiers).  The return status is tracked via ?.

     Decrypt   [Option] For unencrypted messages this command is identical to Copy; Encrypted messages are
               first decrypted, if possible, and then copied.

     decrypt   [Option] For unencrypted messages this command is identical to copy; Encrypted messages are
               first decrypted, if possible, and then copied.

     define, undefine
               The latter command deletes the given macro, the special name ‘*’ will discard all existing
               macros.  Deletion of (a) macro(s) can be performed from within running (a) macro(s), includ‐
               ing self-deletion.  Without arguments the former command prints the current list of macros,
               including their content, otherwise it defines a macro, replacing an existing one of the same
               name as applicable.

               A defined macro can be invoked explicitly by using the call, call_if and xcall commands, or
               implicitly if a macro hook is triggered, e.g., a folder-hook.  Execution of a macro body can
               be stopped from within by calling return.

               Temporary macro block-scope variables can be created or deleted with the local command modi‐
               fier in conjunction with the commands set and unset, respectively.  To enforce unrolling of
               changes made to (global) INTERNAL VARIABLES the command localopts can be used instead; its
               covered scope depends on how (i.e., “as what”: normal macro, folder hook, hook, account
               switch) the macro is invoked.

               Inside a called macro, the given positional parameters are implicitly local to the macro's
               scope, and may be accessed via the variables *, @, # and 1 and any other positive unsigned
               decimal number less than or equal to #.  Positional parameters can be shifted, or become
               completely replaced, removed etc. via vpospar.  A helpful command to perform many sorts of
               number and string evaluations is vexpr.

                     define name {
                       command1
                       command2
                       ...
                       commandN
                     }

                     # E.g.
                     define exmac {
                       echo Parameter 1 of ${#} is ${1}, all: ${*} / ${@}
                       return 1000 0
                     }
                     call exmac Hello macro exmac!
                     echo ${?}/${!}/${^ERRNAME}

     delete, undelete
               (d, u) Marks the given message list as being or not being ‘deleted’, respectively; if no ar‐
               gument has been specified then the usual search for a visible message is performed, as docu‐
               mented for Message list arguments, showing only the next input prompt if the search fails.
               Deleted messages will neither be saved in the secondary mailbox MBOX nor will they be avail‐
               able for most other commands.  If the autoprint variable is set, the new “dot” or the last
               message restored, respectively, is automatically typed; also see dp, dt.

     digmsg    [Only new quoting rules] Digging (information out of) messages is possible through digmsg
               objects, which can be created for the given message number; in compose mode the hyphen-minus
               ‘-’ will instead open the message that is being composed.  If a hyphen-minus is given as the
               optional third argument then output will be generated on the standard output channel instead
               of being subject to consumation by the read or readall commands.

               The objects may be removed again by giving the same identifier used for creation; this step
               could be omitted: objects will be automatically closed when the active mailbox or the com‐
               pose mode is left, respectively.  In all other cases the second argument is an object iden‐
               tifier, and the third and all following arguments are interpreted as via ~^ (see COMMAND
               ESCAPES):

                     ? vput = msgno; digmsg create $msgno
                     ? digmsg $msgno header list;   readall x;   echon $x
                     210 Subject From To Message-ID References In-Reply-To Status
                     ? digmsg $msgno header show Status;readall x;echon $x
                     212 Status
                     RO

                     ? digmsg remove $msgno

     discard   (di) Identical to ignore.  Superseded by the multiplexer headerpick.

     dp, dt    Delete the given messages and automatically type the new “dot” if one exists, regardless of
               the setting of autoprint.

     dotmove   Move the “dot” up or down by one message when given ‘+’ or ‘-’ argument, respectively.

     draft, undraft
               Take message lists and mark each given message as being draft, or not being draft, respec‐
               tively, as documented in the section Message states.

     echo      [Only new quoting rules] (ec) Echoes arguments to standard output and writes a trailing new‐
               line, whereas the otherwise identical echon does not.  Shell-style argument quoting is used,
               Filename transformations are applied to the expanded arguments.  This command also supports
               vput as documented in Command modifiers, and manages the error number !: if data is stored
               in a variable then the return value reflects the length of the result string in case of suc‐
               cess and is ‘-1’ on error.

     echoerr   [Only new quoting rules] Identical to echo except that is echoes to standard error.  Also
               see echoerrn.  In interactive sessions the [Option]al message ring queue for errors will be
               used instead, if available and vput was not used.

     echon     [Only new quoting rules] Identical to echo, but does not write or store a trailing newline.

     echoerrn  [Only new quoting rules] Identical to echoerr, but does not write or store a trailing new‐
               line.

     edit      (e) Point the text EDITOR at each message from the given list in turn.  Modified contents
               are discarded unless the writebackedited variable is set, and are not used unless the mail‐
               box can be written to and the editor returns a successful exit status.  visual can be used
               instead for a more display oriented editor.

     elif      Part of the if (see there for more), elif, else, endif conditional — if the condition of a
               preceding if was false, check the following condition and execute the following block if it
               evaluates true.

     else      (el) Part of the if (see there for more), elif, else, endif conditional — if none of the
               conditions of the preceding if and elif commands was true, the else block is executed.

     endif     (en) Marks the end of an if (see there for more), elif, else, endif conditional execution
               block.

     environ   [Only new quoting rules] S-nail has a strict notion about which variables are INTERNAL
               VARIABLES and which are managed in the program ENVIRONMENT.  Since some of the latter are a
               vivid part of S-nails functioning, however, they are transparently integrated into the nor‐
               mal handling of internal variables via set and unset.  To integrate other environment vari‐
               ables of choice into this transparent handling, and also to export internal variables into
               the process environment where they normally are not, a ‘link’ needs to become established
               with this command, as in, e.g.,

                     environ link PERL5LIB TZ

               Afterwards changing such variables with set will cause automatic updates of the program en‐
               vironment, and therefore be inherited by newly created child processes.  Sufficient system
               support provided (it was in BSD as early as 1987, and is standardized since Y2K) removing
               such variables with unset will remove them also from the program environment, but in any way
               the knowledge they ever have been ‘link’ed will be lost.  Note that this implies that
               localopts may cause loss of such links.

               The command ‘unlink’ will remove an existing link, but leaves the variables as such intact.
               Additionally the subcommands ‘set’ and ‘unset’ are provided, which work exactly the same as
               the documented commands set and unset, but (additionally un)link the variable(s) with the
               program environment and thus immediately export them to, or remove them from (if possible),
               respectively, the program environment.

     errors    [Option] Since S-nail uses the console as a user interface it can happen that messages
               scroll by too fast to become recognized.  An error message ring queue is available which
               stores duplicates of any error message and notifies the user in interactive sessions when‐
               ever a new error has occurred.  The queue is finite: if its maximum size is reached any new
               message replaces the eldest.  The command errors can be used to manage this message queue:
               if given show or no argument the queue will be displayed and cleared, clear will only clear
               all messages from the queue.

     eval      [Only new quoting rules] Construct a command by concatenating the arguments, separated with
               a single space character, and then evaluate the result.  This command passes through the
               exit status ? and error number ! of the evaluated command; also see call.

                     define xxx {
                       echo "xxx arg <$1>"
                       shift
                       if [ $# -gt 0 ]
                         \xcall xxx "$@"
                       endif
                     }
                     define yyy {
                       eval "$@ ' ball"
                     }
                     call yyy '\call xxx' "b\$'\t'u ' "
                     call xxx arg <b      u>
                     call xxx arg <  >
                     call xxx arg <ball>

     exit      (ex or x) Exit from S-nail without changing the active mailbox and skip any saving of mes‐
               sages in the secondary mailbox MBOX, as well as a possibly tracked line editor history-file.
               The optional status number argument will be passed through to exit(3).  [v15 behaviour may
               differ] For now it can happen that the given status will be overwritten, later this will
               only occur if a later error needs to be reported onto an otherwise success indicating sta‐
               tus.

     File      (Fi) Like file, but open the mailbox read-only.

     file      (fi) The file command switches to a new mailbox.  Without arguments it shows status informa‐
               tion of the current mailbox.  If an argument is given, it will write out changes (such as
               deletions) the user has made, open a new mailbox, update the internal variables
               mailbox-resolved and mailbox-display, execute an according folder-hook, if one is installed,
               and optionally display a summary of headers if the variable header is set.

               Filename transformations will be applied to the name argument, and ‘protocol://’ prefixes
               are, i.e., URL syntax is understood, e.g., ‘mbox:///tmp/mdirbox’: if a protocol prefix is
               used the mailbox type is fixated and neither the auto-detection (read on) nor the newfolders
               mechanisms apply.  [Option]ally URLs can also be used to access network resources, which may
               be accessed securely via Encrypted network communication if so supported, and it is possible
               to proxy all network traffic over a SOCKS5 server given via socks-proxy.

                     [v15-compat] protocol://[user[:password]@]host[:port][/path]
                     [no v15-compat] protocol://[user@]host[:port][/path]

               [Option]ally supported network protocols are pop3 (POP3) and pop3s (POP3 with TLS encrypted
               transport), imap and imaps.  The [/path] part is valid only for IMAP; there it defaults to
               INBOX.  Network URLs require a special encoding as documented in the section On URL syntax
               and credential lookup.

               If the resulting file protocol (MBOX database) name is located on a local filesystem then
               the list of all registered filetypes is traversed in order to see whether a transparent in‐
               termediate conversion step is necessary to handle the given mailbox, in which case S-nail
               will use the found hook to load and save data into and from a temporary file, respectively.
               Changing hooks will not affect already opened mailboxes.  For example, the following creates
               hooks for the gzip(1) compression tool and a combined compressed and encrypted format:

                     ? filetype \
                         gzip 'gzip -dc' 'gzip -c' \
                         zst.pgp 'gpg -d | zstd -dc' 'zstd -19 -zc | gpg -e'

               MBOX databases will always be protected via file-region locks (fcntl(2)) during file opera‐
               tions in order to avoid inconsistencies due to concurrent modifications.  [Option] In addi‐
               tion mailbox files treated as the system inbox (MAIL), as well as primary system mailboxes
               in general will also be protected by so-called dotlock files, the traditional way of mail
               spool file locking: for any file ‘x’ a lock file ‘x.lock’ will be created for the duration
               of the synchronization — as necessary an external privileged dotlock helper will be used to
               create the dotlock file in the same directory and with the same user and group identities as
               the file of interest.  dotlock-disable can be used to turn off additional dotlock files,
               shall the need arise.

               S-nail by default uses tolerant POSIX rules when reading MBOX database files, but it will
               detect invalid message boundaries in this mode and complain (even more with debug) if any is
               seen: in this case mbox-rfc4155 can be used to create a valid MBOX database from the invalid
               input.

               [Option] If no protocol has been fixated, and name refers to a directory with the subdirec‐
               tories ‘tmp’, ‘new’ and ‘cur’, then it is treated as a folder in “Maildir” format.  The
               maildir format stores each message in its own file, and has been designed so that file lock‐
               ing is not necessary when reading or writing files.

               [v15 behaviour may differ] If no protocol has been fixated and no existing file has been
               found, the variable newfolders controls the format of mailboxes yet to be created.

     filetype, unfiletype
               [Only new quoting rules] Define or list, and remove, respectively, file handler hooks, which
               provide (shell) commands that enable S-nail to load and save MBOX files from and to files
               with the registered file extensions; it will use an intermediate temporary file to store the
               plain data.  The latter command removes the hooks for all given extensions, ‘*’ will remove
               all existing handlers.

               When used without arguments the former shows a list of all currently defined file hooks,
               with one argument the expansion of the given alias.  Otherwise three arguments are expected,
               the first specifying the file extension for which the hook is meant, and the second and
               third defining the load- and save commands, respectively, to deal with the file type, both
               of which must read from standard input and write to standard output.  Changing hooks will
               not affect already opened mailboxes ([v15 behaviour may differ] except below).  [v15 behav‐
               iour may differ] For now too much work is done, and files are oftened read in twice where
               once would be sufficient: this can cause problems if a filetype is changed while such a file
               is opened; this was already so with the built-in support of .gz etc. in Heirloom, and will
               vanish in v15.  [v15 behaviour may differ] For now all handler strings are passed to the
               SHELL for evaluation purposes; in the future a ‘!’ prefix to load and save commands may mean
               to bypass this shell instance: placing a leading space will avoid any possible misinterpre‐
               tations.

                     ? filetype bz2 'bzip2 -dc' 'bzip2 -zc' \
                         gz 'gzip -dc' 'gzip -c'  xz 'xz -dc' 'xz -zc' \
                         zst 'zstd -dc' 'zstd -19 -zc' \
                         zst.pgp 'gpg -d | zstd -dc' 'zstd -19 -zc | gpg -e'
                     ? set record=+sent.zst.pgp

     flag, unflag
               Take message lists and mark the messages as being flagged, or not being flagged, respec‐
               tively, for urgent/special attention.  See the section Message states.

     folder    (fold) The same as file.

     folders   With no arguments, list the names of the folders in the folder directory.  With an existing
               folder as an argument, lists the names of folders below the named folder.

     Followup  (F) Similar to Respond, but saves the message in a file named after the local part of the
               first recipient's address (instead of in record).

     followup  (fo) Similar to respond, but saves the message in a file named after the local part of the
               first recipient's address (instead of in record).

     followupall
               Similar to followup, but responds to all recipients regardless of the flipr variable.

     followupsender
               Similar to Followup, but responds to the sender only regardless of the flipr variable.

     Forward   Similar to forward, but saves the message in a file named after the local part of the recip‐
               ient's address (instead of in record).

     forward   Takes a message and the address of a recipient and forwards the message to him.  The text of
               the original message is included in the new one, with the value of the forward-inject-head
               variable preceding, and the value of forward-inject-tail succeeding it.  To filter the in‐
               cluded header fields to the desired subset use the ‘forward’ slot of the white- and black‐
               listing command headerpick.  Only the first part of a multipart message is included unless
               forward-as-attachment, and recipient addresses will be stripped from comments, names etc.
               unless the internal variable fullnames is set.

               This may generate the errors ^ERR-DESTADDRREQ if no receiver has been specified, ^ERR-PERM
               if some addressees where rejected by expandaddr, ^ERR-NODATA if no applicable messages have
               been given, ^ERR-NOTSUP if multiple messages have been specified, ^ERR-IO if an I/O error
               occurs, ^ERR-NOTSUP if a necessary character set conversion fails, and ^ERR-INVAL for other
               errors.

     from      (f) Takes a list of message specifications and displays a summary of their message headers,
               exactly as via headers, making the first message of the result the new “dot” (the last mes‐
               sage if showlast is set).  An alias of this command is search.  Also see Specifying
               messages.

     Fwd       [Obsolete] Alias for Forward.

     fwd       [Obsolete] Alias for forward.

     fwdignore
               [Obsolete] Superseded by the multiplexer headerpick.

     fwdretain
               [Obsolete] Superseded by the multiplexer headerpick.

     ghost, unghost
               [Obsolete] Replaced by commandalias, uncommandalias.

     headerpick, unheaderpick
               [Only new quoting rules] Multiplexer command to manage white- and blacklisting selections of
               header fields for a variety of applications.  Without arguments the set of contexts that
               have settings is displayed.  When given arguments, the first argument is the context to
               which the command applies, one of (case-insensitive) ‘type’ for display purposes (via, e.g.,
               type), ‘save’ for selecting which headers shall be stored persistently when save, copy, move
               or even decrypting messages (note that MIME related etc. header fields should not be ignored
               in order to not destroy usability of the message in this case), ‘forward’ for stripping down
               messages when forwarding message (has no effect if forward-as-attachment is set), and ‘top’
               for defining user-defined set of fields for the command top.

               The current settings of the given context are displayed if it is the only argument.  A sec‐
               ond argument denotes the type of restriction that is to be chosen, it may be (a case-insen‐
               sitive prefix of) ‘retain’ or ‘ignore’ for white- and blacklisting purposes, respectively.
               Establishing a whitelist suppresses inspection of the corresponding blacklist.

               If no further argument is given the current settings of the given type will be displayed,
               otherwise the remaining arguments specify header fields, which [Option]ally may be given as
               regular expressions, to be added to the given type.  The special wildcard field (asterisk,
               ‘*’) will establish a (fast) shorthand setting which covers all fields.

               The latter command always takes three or more arguments and can be used to remove selec‐
               tions, i.e., from the given context, the given type of list, all the given headers will be
               removed, the special argument ‘*’ will remove all headers.

     headers   (h) Show the current group of headers, the size of which depends on the variable screen in
               interactive mode, and the format of which can be defined with headline.  If a message-speci‐
               fication is given the group of headers containing the first message therein is shown and the
               message at the top of the screen becomes the new “dot”; the last message is targeted if
               showlast is set.

     help      (hel) A synonym for ?.

     history   [Option] Without arguments or when given show all history entries are shown (this mode also
               supports a more verbose output).  load will replace the list of entries with the content of
               history-file, and save will dump the current list to said file, replacing former content.
               clear will delete all history entries.  The argument can also be a signed decimal NUMBER,
               which will select and evaluate the respective history entry, and move it to the top of the
               history; a negative number is used as an offset to the current command, e.g., ‘-1’ will se‐
               lect the last command, the history top.  Please see On terminal control and line editor for
               more on this topic.

     hold      (ho, also preserve) Takes a message list and marks each message therein to be saved in the
               user's system inbox instead of in the secondary mailbox MBOX.  Does not override the delete
               command.  S-nail deviates from the POSIX standard with this command, because a next command
               issued after hold will display the following message, not the current one.

     if        (i) Part of the if, elif, else, endif conditional execution construct — if the given condi‐
               tion is true then the encapsulated block is executed.  The POSIX standard only supports the
               (case-insensitive) conditions ‘r’eceive and ‘s’end, the remaining are non-portable exten‐
               sions.  [v15 behaviour may differ] These commands do not yet use Shell-style argument
               quoting and therefore do not know about input tokens, so that syntax elements have to be
               surrounded by whitespace; in v15 S-nail will inspect all conditions bracket group wise and
               consider the tokens, representing values and operators, therein, which also means that vari‐
               ables will already have been expanded at that time (just like in the shell).

                     if receive
                       commands ...
                     else
                       commands ...
                     endif

               The (case-insensitive) condition ‘t’erminal will evaluate to true if the standard input is a
               terminal, i.e., in interactive sessions.  Another condition can be any boolean value (see
               the section INTERNAL VARIABLES for textual boolean representations) to mark an enwrapped
               block as “never execute” or “always execute”.  (It shall be remarked that a faulty condition
               skips all branches until endif.)

               ([v15 behaviour may differ] In v15 Shell-style argument quoting will be used, and this com‐
               mand will simply interpret expanded tokens.)  It is possible to check INTERNAL VARIABLES as
               well as ENVIRONMENT variables for existence or compare their expansion against a user given
               value or another variable by using the ‘$’ (“variable next”) conditional trigger character;
               a variable on the right hand side may be signalled using the same mechanism.  Variable names
               may be enclosed in a pair of matching braces.  When this mode has been triggered, several
               operators are available:

               Integer operators treat the arguments on the left and right hand side of the operator as in‐
               tegral numbers and compare them arithmetically.  It is an error if any of the operands is
               not a valid integer, an empty argument (which implies it had been quoted) is treated as if
               it were 0.  Available operators are ‘-lt’ (less than), ‘-le’ (less than or equal to), ‘-eq’
               (equal), ‘-ne’ (not equal), ‘-ge’ (greater than or equal to), and ‘-gt’ (greater than).

               String data operators compare the left and right hand side according to their textual con‐
               tent.  Unset variables are treated as the empty string.  The behaviour of string operators
               can be adjusted by prefixing the operator with the modifier trigger commercial at ‘@’, fol‐
               lowed by none to multiple modifiers: for now supported is ‘i’, which turns the comparison
               into a case-insensitive one: this is implied if no modifier follows the trigger.

               Available string operators are ‘<’ (less than), ‘<=’ (less than or equal to), ‘==’ (equal),
               ‘!=’ (not equal), ‘>=’ (greater than or equal to), ‘>’ (greater than), ‘=%’ (is substring
               of) and ‘!%’ (is not substring of).  By default these operators work on bytes and (there‐
               fore) do not take into account character set specifics.  If the case-insensitivity modifier
               has been used, case is ignored according to the rules of the US-ASCII encoding, i.e., bytes
               are still compared.

               When the [Option]al regular expression support is available, the additional string operators
               ‘=~’ and ‘!~’ can be used.  They treat the right hand side as an extended regular expression
               that is matched according to the active locale (see Character sets), i.e., character sets
               should be honoured correctly.

               Conditions can be joined via AND-OR lists (where the AND operator is ‘&&’ and the OR opera‐
               tor is ‘||’), which have equal precedence and will be evaluated with left associativity,
               thus using the same syntax that is known for the sh(1).  It is also possible to form groups
               of conditions and lists by enclosing them in pairs of brackets ‘[ ... ]’, which may be in‐
               terlocked within each other, and also be joined via AND-OR lists.

               The results of individual conditions and entire groups may be modified via unary operators:
               the unary operator ‘!’ will reverse the result.

                     # (This not in v15, there [ -n "$debug"]!)
                     if $debug
                       echo *debug* is set
                     endif
                     if [ "$ttycharset" == UTF-8 ] || \
                         [ "$ttycharset" @i== UTF8 ]
                       echo *ttycharset* is UTF-8, the former case-sensitive!
                     endif
                     set t1=one t2=one
                     if [ "${t1}" == "${t2}" ]
                       echo These two variables are equal
                     endif
                     if [ "$features" =% +regex ] && \
                         [ "$TERM" @i=~ "^xterm.*" ]
                       echo ..in an X terminal
                     endif
                     if [ [ true ] && [ [ "${debug}" != '' ] || \
                         [ "$verbose" != '' ] ] ]
                       echo Noisy, noisy
                     endif
                     if true && [ "$debug" != '' ] || [ "${verbose}" != '' ]
                       echo Left associativity, as is known from the shell
                     endif

     ignore    (ig) Identical to discard.  Superseded by the multiplexer headerpick.

     list      Shows the names of all available commands, alphabetically sorted.  If given any non-white‐
               space argument the list will be shown in the order in which command prefixes are searched.
               [Option] In conjunction with a set variable verbose additional information will be provided
               for each command: the argument type will be indicated, the documentation string will be
               shown, and the set of command flags will show up:

               ‘`local'’    command supports the command modifier local.
               ‘`vput'’     command supports the command modifier vput.
               ‘*!*’        the error number is tracked in !.
               ‘needs-box’  whether the command needs an active mailbox, a file.
               ‘ok:’        indicators whether command is ...
                            ‘batch/interactive’
                                          usable in interactive or batch mode (-#).
                            ‘send-mode’   usable in send mode.
                            ‘subprocess’  allowed to be used when running in a subprocess instance, e.g.,
                                          from within a macro that is called via on-compose-splice.
               ‘not ok:’    indicators whether command is not ...
                            ‘compose-mode’  available in compose mode.
                            ‘startup’       available during program startup, e.g., in Resource files.
               ‘gabby’      The command produces history-gabby history entries.

     localopts
               This command can be used to localize changes to (linked) ENVIRONMENT as well as (global)
               INTERNAL VARIABLES, meaning that their state will be reverted to the former one once the
               “covered scope” is left.  Just like the command modifier local, which provides block-scope
               localization for some commands (instead), it can only be used inside of macro definition
               blocks introduced by account or define.  The covered scope of an account is left once a dif‐
               ferent account is activated, and some macros, notably folder-hooks, use their own specific
               notion of covered scope, here it will be extended until the folder is left again.

               This setting stacks up: i.e., if ‘macro1’ enables change localization and calls ‘macro2’,
               which explicitly resets localization, then any value changes within ‘macro2’ will still be
               reverted when the scope of ‘macro1’ is left.  (Caveats: if in this example ‘macro2’ changes
               to a different account which sets some variables that are already covered by localizations,
               their scope will be extended, and in fact leaving the account will (thus) restore settings
               in (likely) global scope which actually were defined in a local, macro private context!)

               This command takes one or two arguments, the optional first one specifies an attribute that
               may be one of scope, which refers to the current scope and is thus the default, call, which
               causes any macro that is being called to be started with localization enabled by default, as
               well as call-fixate, which (if enabled) disallows any called macro to turn off localization:
               like this it can be ensured that once the current scope regains control, any changes made in
               deeper levels have been reverted.  The latter two are mutually exclusive, and neither af‐
               fects xcall.  The (second) argument is interpreted as a boolean (string, see INTERNAL
               VARIABLES) and states whether the given attribute shall be turned on or off.

                     define temporary_settings {
                       set possibly_global_option1
                       localopts on
                       set localized_option1
                       set localized_option2
                       localopts scope off
                       set possibly_global_option2
                     }

     Lreply    Reply to messages that come in via known (mlist) or subscribed (mlsubscribe) mailing lists,
               or pretend to do so (see Mailing lists): on top of the usual reply functionality this will
               actively resort and even remove message recipients in order to generate a message that is
               supposed to be sent to a mailing list.  For example it will also implicitly generate a
               ‘Mail-Followup-To:’ header if that seems useful, regardless of the setting of the variable
               followup-to.  For more documentation please refer to On sending mail, and non-interactive
               mode.

               This may generate the errors ^ERR-DESTADDRREQ if no receiver has been specified, ^ERR-PERM
               if some addressees where rejected by expandaddr, ^ERR-NODATA if no applicable messages have
               been given, ^ERR-IO if an I/O error occurs, ^ERR-NOTSUP if a necessary character set conver‐
               sion fails, and ^ERR-INVAL for other errors.  Occurrence of some of the errors depend on the
               value of expandaddr.  Any error stops processing of further messages.

     Mail      Similar to mail, but saves the message in a file named after the local part of the first re‐
               cipient's address (instead of in record).

     mail      (m) Takes a (list of) recipient address(es) as (an) argument(s), or asks on standard input
               if none were given; then collects the remaining mail content and sends it out.  Unless the
               internal variable fullnames is set recipient addresses will be stripped from comments, names
               etc.  For more documentation please refer to On sending mail, and non-interactive mode.

               This may generate the errors ^ERR-DESTADDRREQ if no receiver has been specified, ^ERR-PERM
               if some addressees where rejected by expandaddr, ^ERR-NODATA if no applicable messages have
               been given, ^ERR-NOTSUP if multiple messages have been specified, ^ERR-IO if an I/O error
               occurs, ^ERR-NOTSUP if a necessary character set conversion fails, and ^ERR-INVAL for other
               errors.  Occurrence of some of the errors depend on the value of expandaddr.

     mbox      (mb) The given message list is to be sent to the secondary mailbox MBOX when S-nail is quit;
               this is the default action unless the variable hold is set.  [v15 behaviour may differ] This
               command can only be used in a primary system mailbox.

     mimetype, unmimetype
               Without arguments the content of the MIME type cache will displayed; a more verbose listing
               will be produced if either of debug or verbose are set.  When given arguments they will be
               joined, interpreted as shown in The mime.types files (also see HTML mail and MIME
               attachments), and the resulting entry will be added (prepended) to the cache.  In any event
               MIME type sources are loaded first as necessary – mimetypes-load-control can be used to
               fine-tune which sources are actually loaded.

               The latter command deletes all specifications of the given MIME type, thus ‘? unmimetype
               text/plain’ will remove all registered specifications for the MIME type ‘text/plain’.  The
               special name ‘*’ will discard all existing MIME types, just as will ‘reset’, but which also
               reenables cache initialization via mimetypes-load-control.

     mlist, unmlist
               The latter command removes all given mailing lists, the special name ‘*’ can be used to re‐
               move all registered lists.  The former will list all currently defined mailing lists (and
               their attributes, if any) when used without arguments; a more verbose listing will be pro‐
               duced if either of debug or verbose are set.  Otherwise all given arguments will be added
               and henceforth be recognized as mailing lists.  If the [Option]al regular expression support
               is available then any argument which contains any of the “magical” regular expression char‐
               acters ‘^[]*+?|$’ (see re_format(7)) will be interpreted as one, which allows matching of
               many addresses with a single expression.  The mlsubscribe pair of commands manages subscrip‐
               tion attributes of mailing lists.

     mimeview  [v15 behaviour may differ] Only available in interactive mode, this command allows one to
               display MIME parts which require external MIME handler programs to run which do not inte‐
               grate in S-nails normal type output (see HTML mail and MIME attachments).  ([v15 behaviour
               may differ] No syntax to directly address parts, this restriction may vanish.)  The user
               will be asked for each non-text part of the given message in turn whether the registered
               handler shall be used to display the part.

     mlsubscribe, unmlsubscribe
               The latter command removes the subscription attribute from all given mailing lists, the spe‐
               cial name ‘*’ can be used to do so for any registered list.  The former will list all cur‐
               rently defined mailing lists which have a subscription attribute when used without argu‐
               ments; a more verbose listing will be produced if either of debug or verbose are set.  Oth‐
               erwise this attribute will be set for all given mailing lists, newly creating them as neces‐
               sary (as via mlist).  Also see followup-to.

     Move      Similar to move, but moves the messages to a file named after the local part of the sender
               address of the first message (instead of in record).

     move      Acts like copy but marks the messages for deletion if they were transferred successfully.

     More      Like more, but also displays header fields which would not pass the headerpick selection,
               and all MIME parts.  Identical to Page.

     more      Invokes the PAGER on the given messages, even in non-interactive mode and as long as the
               standard output is a terminal.  Identical to page.

     netrc     [Option] When used without arguments or if show has been given the content of the ~/.netrc
               cache is shown, loading it first as necessary.  If the argument is load then the cache will
               only be initialized and clear will remove its contents.  Note that S-nail will try to load
               the file only once, use ‘netrc clear’ to unlock further attempts.  See netrc-lookup,
               netrc-pipe and the section On URL syntax and credential lookup; the section The .netrc file
               documents the file format in detail.

     newmail   Checks for new mail in the current folder without committing any changes before.  If new
               mail is present, a message is shown.  If the header variable is set, the headers of each new
               message are also shown.  This command is not available for all mailbox types.

     next      (n) (like ‘+’ or “ENTER”) Goes to the next message in sequence and types it.  With an argu‐
               ment list, types the next matching message.

     New       Same as Unread.

     new       Same as unread.

     noop      If the current folder is accessed via a network connection, a “NOOP” command is sent, other‐
               wise no operation is performed.

     Page      Like page, but also displays header fields which would not pass the headerpick selection,
               and all MIME parts.  Identical to More.

     page      Invokes the PAGER on the given messages, even in non-interactive mode and as long as the
               standard output is a terminal.  Identical to more.

     Pipe      Like pipe but also pipes header fields which would not pass the headerpick selection, and
               all parts of MIME ‘multipart/alternative’ messages.

     pipe      (pi) Takes an optional message list and shell command (that defaults to cmd), and pipes the
               messages through the command.  If the page variable is set, every message is followed by a
               formfeed character.

     preserve  (pre) A synonym for hold.

     Print     (P) Alias for Type.

     print     (p) Research UNIX equivalent of type.

     quit      (q) Terminates the session, saving all undeleted, unsaved messages in the current secondary
               mailbox MBOX, preserving all messages marked with hold or preserve or never referenced in
               the system inbox, and removing all other messages from the primary system mailbox.  If new
               mail has arrived during the session, the message “You have new mail” will be shown.  If
               given while editing a mailbox file with the command line option -f, then the edit file is
               rewritten.  A return to the shell is effected, unless the rewrite of edit file fails, in
               which case the user can escape with the exit command.  The optional status number argument
               will be passed through to exit(3).  [v15 behaviour may differ] For now it can happen that
               the given status will be overwritten, later this will only occur if a later error needs to
               be reported onto an otherwise success indicating status.

     read      [Only new quoting rules] Read a line from standard input, or the channel set active via
               readctl, and assign the data, which will be split as indicated by ifs, to the given vari‐
               ables.  The variable names are checked by the same rules as documented for vput, and the
               same error codes will be seen in !; the exit status ? indicates the number of bytes read, it
               will be ‘-1’ with the error number ! set to ^ERR-BADF in case of I/O errors, or ^ERR-NONE
               upon End-Of-File.  If there are more fields than variables, assigns successive fields to the
               last given variable.  If there are less fields than variables, assigns the empty string to
               the remains.

                     ? read a b c
                        H  e  l  l  o
                     ? echo "<$a> <$b> <$c>"
                     <H> <e> <l  l  o>
                     ? wysh set ifs=:; read a b c;unset ifs
                     hey2.0,:"'you    ",:world!:mars.:
                     ? echo $?/$^ERRNAME / <$a><$b><$c>
                     0/NONE / <hey2.0,><"'you    ",><world!:mars.:><><>

     readall   [Only new quoting rules] Read anything from standard input, or the channel set active via
               readctl, and assign the data to the given variable.  The variable name is checked by the
               same rules as documented for vput, and the same error codes will be seen in !; the exit sta‐
               tus ? indicates the number of bytes read, it will be ‘-1’ with the error number ! set to
               ^ERR-BADF in case of I/O errors, or ^ERR-NONE upon End-Of-File.  [v15 behaviour may differ]
               The input data length is restricted to 31-bits.

     readctl   [Only new quoting rules] Manages input channels for read and readall, to be used to avoid
               complicated or impracticable code, like calling read from within a macro in non-interactive
               mode.  Without arguments, or when the first argument is show, a listing of all known chan‐
               nels is printed.  Channels can otherwise be created, and existing channels can be set active
               and removed by giving the string used for creation.

               The channel name is expected to be a file descriptor number, or, if parsing the numeric
               fails, an input file name that undergoes Filename transformations.  E.g. (this example re‐
               quires a modern shell):

                     $ LC_ALL=C printf 'echon "hey, "\nread a\nyou\necho $a' |\
                       LC_ALL=C s-nail -R#
                     hey, you
                     $ LC_ALL=C printf 'echon "hey, "\nread a\necho $a' |\
                       LC_ALL=C 6<<< 'you' s-nail -R#X'readctl create 6'
                     hey, you

     remove    Removes the named files or directories.  Filename transformations including shell pathname
               wildcard pattern expansions (glob(7)) are performed on the arguments.  If a name refer to a
               mailbox, e.g., a Maildir mailbox, then a mailbox type specific removal will be performed,
               deleting the complete mailbox.  The user is asked for confirmation in interactive mode.

     rename    Takes the name of an existing folder and the name for the new folder and renames the first
               to the second one.  Filename transformations including shell pathname wildcard pattern ex‐
               pansions (glob(7)) are performed on both arguments.  Both folders must be of the same type.

     Reply     (R) Identical to reply except that it replies to only the sender of each message of the
               given list, by using the first message as the template to quote, for the ‘Subject:’ etc.;
               setting flipr will exchange this command with reply.

     reply     (r) Take a message and group-responds to it by addressing the sender and all recipients,
               subject to alternates processing.  followup-to, followup-to-honour, reply-to-honour as well
               as recipients-in-cc influence response behaviour.  Unless the internal variable fullnames is
               set recipient addresses will be stripped from comments, names etc.  quote as well as
               quote-as-attachment configure whether responded-to message shall be quoted etc.; setting
               flipr will exchange this command with Reply.  The command Lreply offers special support for
               replying to mailing lists.  For more documentation please refer to On sending mail, and non-
               interactive mode.

               This may generate the errors ^ERR-DESTADDRREQ if no receiver has been specified, ^ERR-PERM
               if some addressees where rejected by expandaddr, ^ERR-NODATA if no applicable messages have
               been given, ^ERR-IO if an I/O error occurs, ^ERR-NOTSUP if a necessary character set conver‐
               sion fails, and ^ERR-INVAL for other errors.  Occurrence of some of the errors depend on the
               value of expandaddr.  Any error stops processing of further messages.

     replyall  Similar to reply, but initiates a group-reply regardless of the value of flipr.

     replysender
               Similar to Reply, but responds to the sender only regardless of the value of flipr.

     Resend    Like resend, but does not add any header lines.  This is not a way to hide the sender's
               identity, but useful for sending a message again to the same recipients.

     resend    Takes a list of messages and a user name and sends each message to the named user.
               ‘Resent-From:’ and related header fields are prepended to the new copy of the message.  Sav‐
               ing in record is only performed if record-resent is set.

               This may generate the errors ^ERR-DESTADDRREQ if no receiver has been specified, ^ERR-PERM
               if some addressees where rejected by expandaddr, ^ERR-NODATA if no applicable messages have
               been given, ^ERR-IO if an I/O error occurs, ^ERR-NOTSUP if a necessary character set conver‐
               sion fails, and ^ERR-INVAL for other errors.  Occurrence of some of the errors depend on the
               value of expandaddr.  Any error stops processing of further messages.

     Respond   Same as Reply.

     respond   Same as reply.

     respondall
               Same as replyall.

     respondsender
               Same as replysender.

     retain    (ret) Superseded by the multiplexer headerpick.

     return    Only available inside the scope of a defined macro or an account, this will stop evaluation
               of any further macro content, and return execution control to the caller.  The two optional
               parameters must be specified as positive decimal numbers and default to the value 0: the
               first argument specifies the signed 32-bit return value (stored in ? [v15 behaviour may dif‐
               fer] and later extended to signed 64-bit), the second the signed 32-bit error number (stored
               in !).  As documented for ? a non-0 exit status may cause the program to exit.

     Save      (S) Similar to save, but saves the messages in a file named after the local part of the
               sender of the first message instead of (in record and) taking a filename argument; the vari‐
               able outfolder is inspected to decide on the actual storage location.

     save      (s) Takes a message list and a filename and appends each message in turn to the end of the
               file.  Filename transformations including shell pathname wildcard pattern expansions
               (glob(7)) is performed on the filename.  If no filename is given, the secondary mailbox MBOX
               is used.  The filename in quotes, followed by the generated character count is echoed on the
               user's terminal.  If editing a primary system mailbox the messages are marked for deletion.
               Filename transformations will be applied.  To filter the saved header fields to the desired
               subset use the ‘save’ slot of the white- and blacklisting command headerpick.

     savediscard
               [Obsolete] Superseded by the multiplexer headerpick.

     saveignore
               [Obsolete] Superseded by the multiplexer headerpick.

     saveretain
               [Obsolete] Superseded by the multiplexer headerpick.

     search    Takes a message specification (list) and displays a header summary of all matching messages,
               as via headers.  This command is an alias of from.  Also see Specifying messages.

     seen      Takes a message list and marks all messages as having been read.

     set, unset
               (se, [Only new quoting rules] uns) The latter command will delete all given global vari‐
               ables, or only block-scope local ones if the local command modifier has been used.  The for‐
               mer, when used without arguments, will show all currently known variables, being more ver‐
               bose if either of debug or verbose is set.  Remarks: this list mode will not automatically
               link-in known ENVIRONMENT variables, but only explicit addressing will, e.g., via varshow,
               using a variable in an if condition or a string passed to echo, explicit setting, as well as
               some program-internal use cases.

               Otherwise the given variables (and arguments) are set or adjusted.  Arguments are of the
               form ‘name=value’ (no space before or after ‘=’), or plain ‘name’ if there is no value,
               i.e., a boolean variable.  If a name begins with ‘no’, as in ‘set nosave’, the effect is the
               same as invoking the unset command with the remaining part of the variable (‘unset save’).
               [v15 behaviour may differ] In conjunction with the wysh (or local) command prefix(es)
               Shell-style argument quoting can be used to quote arguments as necessary.  [v15 behaviour
               may differ] Otherwise quotation marks may be placed around any part of the assignment state‐
               ment to quote blanks or tabs.

               When operating in global scope any ‘name’ that is known to map to an environment variable
               will automatically cause updates in the program environment (unsetting a variable in the en‐
               vironment requires corresponding system support) — use the command environ for further envi‐
               ronmental control.  If the command modifier local has been used to alter the command to work
               in block-scope all variables have values (may they be empty), and creation of names which
               shadow INTERNAL VARIABLES is actively prevented ([v15 behaviour may differ] shadowing of
               linked ENVIRONMENT variables and free-form versions of variable chains is not yet detected).
               Also see varshow and the sections INTERNAL VARIABLES and ENVIRONMENT.

                     ? wysh set indentprefix=' -> '
                     ? wysh set atab=$'' aspace=' ' zero=0

     shcodec   Apply shell quoting rules to the given raw-data arguments.  Supports vput (see Command
               modifiers).  The first argument specifies the operation: [+]e[ncode] or d[ecode] cause shell
               quoting to be applied to the remains of the line, and expanded away thereof, respectively.
               If the former is prefixed with a plus-sign, the quoted result will not be roundtrip enabled,
               and thus can be decoded only in the very same environment that was used to perform the en‐
               code; also see mle-quote-rndtrip.  If the coding operation fails the error number ! is set
               to ^ERR-CANCELED, and the unmodified input is used as the result; the error number may
               change again due to output or result storage errors.

     shell     [Only new quoting rules] (sh) Invokes an interactive version of the shell, and returns its
               exit status.

     shortcut, unshortcut
               Without arguments the list of all currently defined shortcuts is shown, with one argument
               the expansion of the given shortcut.  Otherwise all given arguments are treated as pairs of
               shortcuts and their expansions, creating new or changing already existing shortcuts, as nec‐
               essary.  The latter command will remove all given shortcuts, the special name ‘*’ will re‐
               move all registered shortcuts.

     shift     [Only new quoting rules] Shift the positional parameter stack (starting at 1) by the given
               number (which must be a positive decimal), or 1 if no argument has been given.  It is an er‐
               ror if the value exceeds the number of positional parameters.  If the given number is 0, no
               action is performed, successfully.  The stack as such can be managed via vpospar.  Note this
               command will fail in account and hook macros unless the positional parameter stack has been
               explicitly created in the current context via vpospar.

     show      Like type, but performs neither MIME decoding nor decryption, so that the raw message text
               is shown.

     size      (si) Shows the size in characters of each message of the given message-list.

     sleep     [Only new quoting rules] Sleep for the specified number of seconds (and optionally millisec‐
               onds), by default interruptably.  If a third argument is given the sleep will be uninter‐
               ruptible, otherwise the error number ! will be set to ^ERR-INTR if the sleep has been inter‐
               rupted.  The command will fail and the error number will be ^ERR-OVERFLOW if the given dura‐
               tion(s) overflow the time datatype, and ^ERR-INVAL if the given durations are no valid inte‐
               gers.

     sort, unsort
               The latter command disables sorted or threaded mode, returns to normal message order and, if
               the header variable is set, displays a header summary.  The former command shows the current
               sorting criterion when used without an argument, but creates a sorted representation of the
               current folder otherwise, and changes the next command and the addressing modes such that
               they refer to messages in the sorted order.  Message numbers are the same as in regular
               mode.  If the header variable is set, a header summary in the new order is also displayed.
               Automatic folder sorting can be enabled by setting the autosort variable, as in, e.g., ‘set
               autosort=thread’.  Possible sorting criterions are:

               date     Sort the messages by their ‘Date:’ field, that is by the time they were sent.
               from     Sort messages by the value of their ‘From:’ field, that is by the address of the
                        sender.  If the showname variable is set, the sender's real name (if any) is used.
               size     Sort the messages by their size.
               spam     [Option] Sort the message by their spam score, as has been classified by spamrate.
               status   Sort the messages by their message status.
               subject  Sort the messages by their subject.
               thread   Create a threaded display.
               to       Sort messages by the value of their ‘To:’ field, that is by the address of the re‐
                        cipient.  If the showname variable is set, the recipient's real name (if any) is
                        used.

     source    [Only new quoting rules] (so) The source command reads commands from the given file.
               Filename transformations will be applied.  If the given expanded argument ends with a verti‐
               cal bar ‘|’ then the argument will instead be interpreted as a shell command and S-nail will
               read the output generated by it.  Dependent on the settings of posix and errexit, and also
               dependent on whether the command modifier ignerr had been used, encountering errors will
               stop sourcing of the given input.  [v15 behaviour may differ] Note that source cannot be
               used from within macros that execute as folder-hooks or accounts, i.e., it can only be
               called from macros that were called.

     source_if
               [Only new quoting rules] The difference to source (beside not supporting pipe syntax aka
               shell command input) is that this command will not generate an error nor warn if the given
               file argument cannot be opened successfully.

     spamclear
               [Option] Takes a list of messages and clears their ‘is-spam’ flag.

     spamforget
               [Option] Takes a list of messages and causes the spam-interface to forget it has ever used
               them to train its Bayesian filter.  Unless otherwise noted the ‘is-spam’ flag of the message
               is inspected to chose whether a message shall be forgotten to be “ham” or “spam”.

     spamham   [Option] Takes a list of messages and informs the Bayesian filter of the spam-interface that
               they are “ham”.  This also clears the ‘is-spam’ flag of the messages in question.

     spamrate  [Option] Takes a list of messages and rates them using the configured spam-interface, with‐
               out modifying the messages, but setting their ‘is-spam’ flag as appropriate; because the
               spam rating headers are lost the rate will be forgotten once the mailbox is left.  Refer to
               the manual section Handling spam for the complete picture of spam handling in S-nail.

     spamset   [Option] Takes a list of messages and sets their ‘is-spam’ flag.

     spamspam  [Option] Takes a list of messages and informs the Bayesian filter of the spam-interface that
               they are “spam”.  This also sets the ‘is-spam’ flag of the messages in question.

     thread    [Obsolete] The same as ‘sort thread’ (consider using a ‘commandalias’ as necessary).

     tls       [Only new quoting rules] TLS information and management command multiplexer to aid in
               Encrypted network communication.  Commands support vput if so documented (see Command
               modifiers).  The result that is shown in case of errors is always the empty string, errors
               can be identified via the error number !.  For example, string length overflows are caught
               and set ! to ^ERR-OVERFLOW.  Note this command of course honours the overall TLS configura‐
               tion.

                     ? vput tls result fingerprint pop3s://ex.am.ple
                     ? echo $?/$!/$^ERRNAME: $result

               fingerprint Show the tls-fingerprint-digested fingerprint of the certificate of the given
                         HOST (‘server:port’, where the port defaults to the HTTPS port, 443).
                         tls-fingerprint is actively ignored for the runtime of this command.  Only avail‐
                         able if the term ‘+sockets’ is included in features.

     Top       Like top but always uses the headerpick ‘type’ slot for white- and blacklisting header
               fields.

     top       (to) Takes a message list and types out the first toplines lines of each message on the
               user's terminal.  Unless a special selection has been established for the ‘top’ slot of the
               headerpick command, the only header fields that are displayed are ‘From:’, ‘To:’, ‘CC:’, and
               ‘Subject:’.  Top will always use the ‘type’ headerpick selection instead.  It is possible to
               apply compression to what is displayed by setting topsqueeze.  Messages are decrypted and
               converted to the terminal character set if necessary.

     touch     (tou) Takes a message list and marks the messages for saving in the secondary mailbox MBOX.
               S-nail deviates from the POSIX standard with this command, as a following next command will
               display the following message instead of the current one.

     Type      (T) Like type but also displays header fields which would not pass the headerpick selection,
               and all visualizable parts of MIME ‘multipart/alternative’ messages.

     type      (t) Takes a message list and types out each message on the user's terminal.  The display of
               message headers is selectable via headerpick.  For MIME multipart messages, all parts with a
               content type of ‘text’, all parts which have a registered MIME type handler (see HTML mail
               and MIME attachments) which produces plain text output, and all ‘message’ parts are shown,
               others are hidden except for their headers.  Messages are decrypted and converted to the
               terminal character set if necessary.  The command mimeview can be used to display parts
               which are not displayable as plain text.

     unaccount
               See account.

     unalias   (una) See alias.

     unanswered
               See answered.

     unbind    See bind.

     uncollapse
               See collapse.

     uncolour  See colour.

     undefine  See define.

     undelete  See delete.

     undraft   See draft.

     unflag    See flag.

     unfwdignore
               [Obsolete] Superseded by the multiplexer headerpick.

     unfwdretain
               [Obsolete] Superseded by the multiplexer headerpick.

     unignore  Superseded by the multiplexer headerpick.

     unmimetype
               See mimetype.

     unmlist   See mlist.

     unmlsubscribe
               See mlsubscribe.

     Unread    Same as unread.

     unread    Takes a message list and marks each message as not having been read.

     unretain  Superseded by the multiplexer headerpick.

     unsaveignore
               [Obsolete] Superseded by the multiplexer headerpick.

     unsaveretain
               [Obsolete] Superseded by the multiplexer headerpick.

     unset     [Only new quoting rules] (uns) See set.

     unshortcut
               See shortcut.

     unsort    See short.

     unthread  [Obsolete] Same as unsort.

     urlcodec  Perform URL percent codec operations on the raw-data argument, rather according to RFC 3986.
               Supports vput (see Command modifiers), and manages the error number !.  This is a character
               set agnostic and thus locale dependent operation, and it may decode bytes which are invalid
               in the current ttycharset.  [v15 behaviour may differ] This command does not know about URLs
               beside that.

               The first argument specifies the operation: e[ncode] or d[ecode] perform plain URL percent
               en- and decoding, respectively.  p[ath]enc[ode] and p[ath]dec[ode] perform a slightly modi‐
               fied operation which should be better for pathnames: it does not allow a tilde ‘~’, and will
               neither accept hyphen-minus ‘-’ nor dot ‘’.  as an initial character.  The remains of the
               line form the URL data which is to be converted.  If the coding operation fails the error
               number ! is set to ^ERR-CANCELED, and the unmodified input is used as the result; the error
               number may change again due to output or result storage errors.

     varshow   [Only new quoting rules] This command produces the same output as the listing mode of set,
               including verboseity adjustments, but only for the given variables.

     verify    [Option] Takes a message list and verifies each message.  If a message is not a S/MIME
               signed message, verification will fail for it.  The verification process checks if the mes‐
               sage was signed using a valid certificate, if the message sender's email address matches one
               of those contained within the certificate, and if the message content has been altered.

     version   Shows the version and features of S-nail, as well as the build and running system environ‐
               ment.  This command can produce a more verbose output, and supports vput (see Command
               modifiers).

     vexpr     [Only new quoting rules] Evaluate arguments according to a given operator.  This is a multi‐
               plexer command which can be used to perform signed 64-bit numeric calculations as well as
               byte string and string operations.  It uses polish notation, i.e., the operator is the first
               argument and defines the number and type, and the meaning of the remaining arguments.  An
               empty argument is replaced with a 0 if a number is expected.  Supports vput (see Command
               modifiers).

               The result that is shown in case of errors is always ‘-1’ for usage errors and numeric oper‐
               ations, and the empty string for byte string and string operations; if the latter two fail
               to provide result data for “soft” errors, e.g., when a search operation failed, they also
               set the ! error number to ^ERR-NODATA.  Except when otherwise noted numeric arguments are
               parsed as signed 64-bit numbers, and errors will be reported in the error number ! as the
               numeric error ^ERR-RANGE.

               Numeric operations work on one or two signed 64-bit integers.  Numbers prefixed with ‘0x’ or
               ‘0X’ are interpreted as hexadecimal (base 16) numbers, whereas ‘0’ indicates octal (base 8),
               and ‘0b’ as well as ‘0B’ denote binary (base 2) numbers.  It is possible to use any base in
               between 2 and 36, inclusive, with the ‘BASE#number’ notation, where the base is given as an
               unsigned decimal number, e.g., ‘16#AFFE’ is a different way of specifying a hexadecimal num‐
               ber.  Unsigned interpretation of a number can be enforced by prefixing an ‘u’ (case-insensi‐
               tively), e.g., ‘u-110’; this is not necessary for power-of-two bases (2, 4, 8, 16 and 32),
               which will be interpreted as unsigned by default, but it still makes a difference regarding
               overflow detection and overflow constant.  It is possible to enforce signed interpretation
               by (instead) prefixing a ‘s’ (case-insensitively).

               One integer is expected by assignment (equals sign ‘=’), which does nothing but parsing the
               argument, thus detecting validity and possible overflow conditions, and unary not (tilde
               ‘~’), which creates the bitwise complement.  Two integers are used by addition (plus sign
               ‘+’), subtraction (hyphen-minus ‘-’), multiplication (asterisk ‘*’), division (solidus ‘/’)
               and modulo (percent sign ‘%’), as well as for the bitwise operators logical or (vertical bar
               ‘|’, to be quoted) , bitwise and (ampersand ‘&’, to be quoted) , bitwise xor (circumflex
               ‘^’), the bitwise signed left- and right shifts (‘<<’, ‘>>’), as well as for the unsigned
               right shift ‘>>>’.

               Another numeric operation is pbase, which takes a number base in between 2 and 36, inclu‐
               sive, and will act on the second number given just the same as what equals sign ‘=’ does,
               but the number result will be formatted in the base given.

               All numeric operators can be prefixed with a commercial at ‘@’, e.g., ‘@*’: this will turn
               the operation into a saturated one, which means that overflow errors and division and modulo
               by zero are no longer reported via the exit status, but the result will linger at the mini‐
               mum or maximum possible value, instead of overflowing (or trapping).  This is true also for
               the argument parse step.  For the bitwise shifts, the saturated maximum is 63.  Any caught
               overflow will be reported via the error number ! as ^ERR-OVERFLOW.

                     ? vexpr @- +1 -9223372036854775808
                     ? echo $?/$!/$^ERRNAME

               Character set agnostic string functions have no notion of locale settings and character
               sets.

               file-expand Performs the usual Filename transformations on its argument.

               random    Generates a random string of the given length, or of PATH_MAX bytes (a constant
                         from /usr/include) if the value 0 is given; the random string will be base64url
                         encoded according to RFC 4648, and thus be usable as a (portable) filename.

               Byte string operations work on 8-bit bytes and have no notion of locale settings and charac‐
               ter sets, effectively assuming ASCII data.

               length    Queries the length of the given argument.

               hash      Calculates the Chris Torek hash of the given argument.

               find      Byte-searches in the first for the second argument.  Shows the resulting 0-based
                         offset shall it have been found.

               ifind     Identical to find, but works case-insensitively according to the rules of the
                         ASCII character set.

               substring Creates a substring of its first argument.  The second argument is the 0-based
                         starting offset, a negative one counts from the end; the optional third argument
                         specifies the length of the desired result, a negative length leaves off the given
                         number of bytes at the end of the original string, by default the entire string is
                         used; this operation tries to work around faulty arguments (set verbose for error
                         logs), but reports them via the error number ! as ^ERR-OVERFLOW.

               trim      Trim away whitespace characters from both ends of the argument.

               trim-front Trim away whitespace characters from the begin of the argument.

               trim-end  Trim away whitespace characters from the end of the argument.

               String operations work, sufficient support provided, according to the active user's locale
               encoding and character set (see Character sets).

               makeprint (One-way) Converts the argument to something safely printable on the terminal.

               regex     [Option] A string operation that will try to match the first argument with the
                         regular expression given as the second argument.  If the optional third argument
                         has been given then instead of showing the match offset a replacement operation is
                         performed: the third argument is treated as if specified within dollar-single-
                         quote (see Shell-style argument quoting), and any occurrence of a positional pa‐
                         rameter, e.g., 1, is replaced by the corresponding match group of the regular ex‐
                         pression:

                               ? vput vexpr res regex bananarama \
                                   (.*)NanA(.*) '\${1}au\$2'
                               ? echo $?/$!/$^ERRNAME: $res

               iregex    On otherwise identical case-insensitive equivalent to regex:

                               ? vput vexpr res ire bananarama \
                                   (.*)NanA(.*) '\${1}au\$2'
                               ? echo $?/$!/$^ERRNAME: $res

     vpospar   [Only new quoting rules] Manage the positional parameter stack (see 1, #, *, @ as well as
               shift).  If the first argument is ‘clear’, then the positional parameter stack of the cur‐
               rent context, or the global one, if there is none, is cleared.  If it is ‘set’, then the re‐
               maining arguments will be used to (re)create the stack, if the parameter stack size limit is
               excessed an ^ERR-OVERFLOW error will occur.

               If the first argument is ‘quote’, a round-trip capable representation of the stack contents
               is created, with each quoted parameter separated from each other with the first character of
               ifs, and followed by the first character of if-ws, if that is not empty and not identical to
               the first.  If that results in no separation at all a space character is used.  This mode
               supports vput (see Command modifiers).  I.e., the subcommands ‘set’ and ‘quote’ can be used
               (in conjunction with eval) to (re)create an argument stack from and to a single variable
               losslessly.

                     ? vpospar set hey, "'you    ", world!
                     ? echo $#: <${1}><${2}><${3}>
                     ? vput vpospar x quote
                     ? vpospar clear
                     ? echo $#: <${1}><${2}><${3}>
                     ? eval vpospar set ${x}
                     ? echo $#: <${1}><${2}><${3}>

     visual    (v) Takes a message list and invokes the VISUAL display editor on each message.  Modified
               contents are discarded unless the writebackedited variable is set, and are not used unless
               the mailbox can be written to and the editor returns a successful exit status.  edit can be
               used instead for a less display oriented editor.

     write     (w) For conventional messages the body without all headers is written.  The original message
               is never marked for deletion in the originating mail folder.  The output is decrypted and
               converted to its native format as necessary.  If the output file exists, the text is ap‐
               pended.  If a message is in MIME multipart format its first part is written to the specified
               file as for conventional messages, handling of the remains depends on the execution mode.
               No special handling of compressed files is performed.

               In interactive mode the user is consecutively asked for the filenames of the processed
               parts.  For convience saving of each part may be skipped by giving an empty value, the same
               result as writing it to /dev/null.  Shell piping the part content by specifying a leading
               vertical bar ‘|’ character for the filename is supported.  Other user input undergoes the
               usual Filename transformations, including shell pathname wildcard pattern expansions
               (glob(7)) and shell variable expansion for the message as such, not the individual parts,
               and contents of the destination file are overwritten if the file previously existed.

               [v15 behaviour may differ] In non-interactive mode any part which does not specify a file‐
               name is ignored, and suspicious parts of filenames of the remaining parts are URL percent
               encoded (as via urlcodec) to prevent injection of malicious character sequences, resulting
               in a filename that will be written into the current directory.  Existing files will not be
               overwritten, instead the part number or a dot are appended after a number sign ‘#’ to the
               name until file creation succeeds (or fails due to other reasons).

     xcall     [Only new quoting rules] The sole difference to call is that the new macro is executed in
               place of the current one, which will not regain control: all resources of the current macro
               will be released first.  This implies that any setting covered by localopts will be forgot‐
               ten and covered variables will become cleaned up.  If this command is not used from within a
               called macro it will silently be (a more expensive variant of) call.

     xit       (x) A synonym for exit.

     z         [Only new quoting rules] S-nail presents message headers in screenfuls as described under
               the headers command.  Without arguments this command scrolls to the next window of messages,
               likewise if the argument is ‘+’.  An argument of ‘-’ scrolls to the last, ‘^’ scrolls to the
               first, and ‘$’ to the last screen of messages.  A number argument prefixed by ‘+’ or ‘-’ in‐
               dicates that the window is calculated in relation to the current position, and a number
               without a prefix specifies an absolute position.

     Z         [Only new quoting rules] Similar to z, but scrolls to the next or previous window that con‐
               tains at least one ‘new’ or flagged message.

COMMAND ESCAPES
     Command escapes are available in compose mode, and are used to perform special functions when compos‐
     ing messages.  Command escapes are only recognized at the beginning of lines, and consist of a trigger
     (escape), and a command character.  The actual escape character can be set via the internal variable
     escape, it defaults to the tilde ‘~’.  Otherwise ignored whitespace characters following the escape
     character will prevent a possible addition of the command line to the [Option]al history.

     Unless otherwise noted all compose mode command escapes ensure proper updates of the variables which
     represent the error number ! and the exit status ?.  If the variable errexit is set they will, unless
     stated otherwise, error out message compose mode and cause a program exit if an operation fails; an
     effect equivalent to the command modifier ignerr can however be achieved by placing a hyphen-minus ‘-’
     after (possible whitespace following) the escape character.  If the [Option]al key bindings are avail‐
     able it is possible to create bindings specifically for the compose mode.

     ~~ string
               Insert the string of text in the message prefaced by a single ‘~’.  (If the escape character
               has been changed, that character must be doubled instead.)

     ~! command
               Execute the indicated shell command which follows, replacing unescaped exclamation marks
               with the previously executed command if the internal variable bang is set, then return to
               the message.

     ~.        End compose mode and send the message.  The hooks on-compose-splice-shell and
               on-compose-splice, in order, will be called when set, after which askatend will be checked,
               a set on-compose-leave hook will be called, autocc and autobcc will be joined in if set,
               asksend will be honoured in interactive mode, finally a given message-inject-tail will be
               incorporated, after which the compose mode is left.

     ~: S-nail-command or ~_ S-nail-command
               Execute the given S-nail command.  Not all commands, however, are allowed.

     ~< filename
               Identical to ~r.

     ~<! command
               command is executed using the shell.  Its standard output is inserted into the message.

     ~?        [Option] Write a summary of command escapes.

     ~@ [filename...]
               Append or edit the list of attachments.  Does not manage the error number ! and the exit
               status ? (please use ~^ instead if this is a concern).  The append mode expects a list of
               filename arguments as shell tokens (see Shell-style argument quoting; token-separating com‐
               mas are ignored, too), to be interpreted as documented for the command line option -a, with
               the message number exception as below.

               Without filename arguments the attachment list is edited, entry by entry; if a filename is
               left empty, that attachment is deleted from the list; once the end of the list is reached
               either new attachments may be entered or the session can be quit by committing an empty
               “new” attachment.  In non-interactive mode or in batch mode (-#) the list of attachments is
               effectively not edited but instead recreated; again, an empty input ends list creation.

               For all modes, if a given filename solely consists of the number sign ‘#’ followed by either
               a valid message number of the currently active mailbox, or by a period ‘.’, referring to the
               current message of the active mailbox, the so-called “dot”, then the given message is at‐
               tached as a ‘message/rfc822’ MIME message part.  The number sign must be quoted to avoid
               misinterpretation with the shell comment character.

     ~| command
               Pipe the message text through the specified filter command.  If the command gives no output
               or terminates abnormally, retain the original text of the message.  E.g., the command fmt(1)
               is often used as a rejustifying filter.

               If the first character of the command is a vertical bar, then the entire message including
               header fields is subject to the filter command, e.g., ‘~|| echo Fcc: /tmp/test; cat’ will
               prepend a file-carbon-copy message header.  Also see ~e, ~v.

     ~^ cmd [subcmd [arg3 [arg4]]]
               Low-level compose mode command which shares the semantic with digmsg and is meant for
               scripted message access, i.e., for on-compose-splice and on-compose-splice-shell.  The used
               protocol is likely subject to changes, and therefore the mentioned hooks receive the used
               protocol version as an initial line.  In general the first field of a response line repre‐
               sents a status code which specifies whether a command was successful or not, whether result
               data is to be expected, and if, the format of the result data.  Does not manage the error
               number ! and the exit status ?, because errors are reported via the protocol (hard errors
               like I/O failures cannot be handled).

               This command has read-only access to several virtual pseudo headers in the S-nail private
               namespace which optionally (except for the non-optional first) exist in compose mode:

               ‘Mailx-Command:’
                         The name of the command that generates the message, one of ‘forward’, ‘Lreply’,
                         ‘mail’, ‘Reply’, ‘reply’, ‘resend’.
               ‘Mailx-Raw-To:’
               ‘Mailx-Raw-Cc:’
               ‘Mailx-Raw-Bcc:’
                         Represent the frozen initial state of these headers before any transformation
                         (e.g., alias, alternates, recipients-in-cc etc.) took place.
               ‘Mailx-Orig-From:’
               ‘Mailx-Orig-To:’
               ‘Mailx-Orig-Cc:’
               ‘Mailx-Orig-Bcc:’
                         The values of said headers of the original message which has been addressed by any
                         of reply, forward, resend.

               Error status code lines may optionally contain additional context.  The status codes are:

               ‘210’     Status ok; the remains of the line are the result.
               ‘211’     Status ok; the rest of the line is optionally used for more status.  What follows
                         are lines of result addresses, terminated by an empty line.  The address lines
                         consist of two fields, the first of which is the plain address, e.g.,
                         ‘bob@exam.ple’, separated by a single ASCII SP space from the second which con‐
                         tains the unstripped address, even if that is identical to the first field, e.g.,
                         ‘(Lovely) Bob <bob@exam.ple>’.  Non-network addressees, however, place a single-
                         letter indicating the address type in the first field (hyphen-minus ‘-’ for files,
                         vertical bar ‘|’ for pipes, and number sign ‘#’ for names: what is supposed to be‐
                         come expanded via alias), and only the second field contains a value.  All the in‐
                         put, including the empty line, must be consumed before further commands can be is‐
                         sued.
               ‘212’     Status ok; the rest of the line is optionally used for more status.  What follows
                         are lines of furtherly unspecified string content, terminated by an empty line.
                         All the input, including the empty line, must be consumed before further commands
                         can be issued.
               ‘500’     Syntax error; invalid command.
               ‘501’     Syntax error in parameters or arguments.
               ‘505’     Error: an argument fails verification.  For example an invalid address has been
                         specified (also see expandaddr), or an attempt was made to modify anything in
                         S-nail's own namespace, or a modifying subcommand has been used on a read-only
                         message.
               ‘506’     Error: an otherwise valid argument is rendered invalid due to context.  For exam‐
                         ple, a second address is added to a header which may consist of a single address
                         only.

               If a command indicates failure then the message will have remained unmodified.  Most com‐
               mands can fail with ‘500’ if required arguments are missing (false command usage).  The fol‐
               lowing (case-insensitive) commands are supported:

               version  This command will print the protocol version via 210.

               header   This command allows listing, inspection, and editing of message headers.  Header
                        name case is not normalized, and case-insensitive comparison should be used when
                        matching names.  The second argument specifies the subcommand to apply, one of:

                        list      Without a third argument a list of all yet existing headers is given via
                                  ‘210’; this command is the default command of header if no second argu‐
                                  ment has been given.  A third argument restricts output to the given
                                  header only, which may fail with ‘501’ if no such field is defined.

                        show      Shows the content of the header given as the third argument.  Dependent
                                  on the header type this may respond with ‘211’ or ‘212’; any failure re‐
                                  sults in ‘501’.

                        remove    This will remove all instances of the header given as the third argument,
                                  reporting ‘210’ upon success, ‘501’ if no such header can be found, and
                                  Ql 505 on S-nail namespace violations.

                        remove-at This will remove from the header given as the third argument the instance
                                  at the list position (counting from one!) given with the fourth argument,
                                  reporting ‘210’ upon success or ‘505’ if the list position argument is
                                  not a number or on S-nail namespace violations, and ‘501’ if no such
                                  header instance exists.

                        insert    Create a new or an additional instance of the header given in the third
                                  argument, with the header body content as given in the fourth argument
                                  (the remains of the line).  It may return ‘501’ if the third argument
                                  specifies a free-form header field name that is invalid, or if body con‐
                                  tent extraction fails to succeed, ‘505’ if any extracted address does not
                                  pass syntax and/or security checks or on S-nail namespace violations, and
                                  ‘506’ to indicate prevention of excessing a single-instance header — note
                                  that ‘Subject:’ can be appended to (a space separator will be added auto‐
                                  matically first).

                                  ‘210’ is returned upon success, followed by the name of the header and
                                  the list position of the newly inserted instance.  The list position is
                                  always 1 for single-instance header fields.  All free-form header fields
                                  are managed in a single list.

               attachment This command allows listing, removal and addition of message attachments.  The
                        second argument specifies the subcommand to apply, one of:

                        list      List all attachments via ‘212’, or report ‘501’ if no attachments exist.
                                  This command is the default command of attachment if no second argument
                                  has been given.

                        remove    This will remove the attachment given as the third argument, and report
                                  ‘210’ upon success or ‘501’ if no such attachment can be found.  If there
                                  exists any path component in the given argument, then an exact match of
                                  the path which has been used to create the attachment is used directly,
                                  but if only the basename of that path matches then all attachments are
                                  traversed to find an exact match first, and the removal occurs after‐
                                  wards; if multiple basenames match, a ‘506’ error occurs.  Message at‐
                                  tachments are treated as absolute pathnames.

                                  If no path component exists in the given argument, then all attachments
                                  will be searched for ‘filename=’ parameter matches as well as for matches
                                  of the basename of the path which has been used when the attachment has
                                  been created; multiple matches result in a ‘506’.

                        remove-at This will interpret the third argument as a number and remove the attach‐
                                  ment at that list position (counting from one!), reporting ‘210’ upon
                                  success or ‘505’ if the argument is not a number or ‘501’ if no such at‐
                                  tachment exists.

                        insert    Adds the attachment given as the third argument, specified exactly as
                                  documented for the command line option -a, and supporting the message
                                  number extension as documented for ~@.  This reports ‘210’ upon success,
                                  with the index of the new attachment following, ‘505’ if the given file
                                  cannot be opened, ‘506’ if an on-the-fly performed character set conver‐
                                  sion fails, otherwise ‘501’ is reported; this is also reported if charac‐
                                  ter set conversion is requested but not available.

                        attribute This uses the same search mechanism as described for remove and prints
                                  any known attributes of the first found attachment via ‘212’ upon success
                                  or ‘501’ if no such attachment can be found.  The attributes are written
                                  as lines of keyword and value tuples, the keyword being separated from
                                  the rest of the line with an ASCII SP space character.

                        attribute-at This uses the same search mechanism as described for remove-at and is
                                  otherwise identical to attribute.

                        attribute-set This uses the same search mechanism as described for remove, and will
                                  assign the attribute given as the fourth argument, which is expected to
                                  be a value tuple of keyword and other data, separated by a ASCII SP space
                                  or TAB tabulator character.  If the value part is empty, then the given
                                  attribute is removed, or reset to a default value if existence of the at‐
                                  tribute is crucial.

                                  It returns via ‘210’ upon success, with the index of the found attachment
                                  following, ‘505’ for message attachments or if the given keyword is in‐
                                  valid, and ‘501’ if no such attachment can be found.  The following key‐
                                  words may be used (case-insensitively):

                                  ‘filename’  Sets the filename of the MIME part, i.e., the name that is
                                              used for display and when (suggesting a name for) saving
                                              (purposes).
                                  ‘content-description’ Associate some descriptive information to the at‐
                                              tachment's content, used in favour of the plain filename by
                                              some MUAs.
                                  ‘content-id’ May be used for uniquely identifying MIME entities in sev‐
                                              eral contexts; this expects a special reference address for‐
                                              mat as defined in RFC 2045 and generates a ‘505’ upon address
                                              content verification failure.
                                  ‘content-type’ Defines the media type/subtype of the part, which is man‐
                                              aged automatically, but can be overwritten.
                                  ‘content-disposition’ Automatically set to the string ‘attachment’.

                        attribute-set-at This uses the same search mechanism as described for remove-at and
                                  is otherwise identical to attribute-set.

     ~A        The same as ‘~i Sign’.

     ~a        The same as ‘~i sign’.

     ~b name ...
               Add the given names to the list of blind carbon copy recipients.

     ~c name ...
               Add the given names to the list of carbon copy recipients.

     ~d        Read the file specified by the DEAD variable into the message.

     ~e        Invoke the text EDITOR on the message collected so far, then return to compose mode.  ~v can
               be used for a more display oriented editor, and ~|| offers a pipe-based editing approach.

     ~F messages
               Read the named messages into the message being sent, including all message headers and MIME
               parts.  If no messages are specified, read in the current message, the “dot”.

     ~f messages
               Read the named messages into the message being sent.  If no messages are specified, read in
               the current message, the “dot”.  Strips down the list of header fields according to the
               ‘type’ white- and blacklist selection of headerpick.  For MIME multipart messages, only the
               first displayable part is included.

     ~H        Edit the message header fields ‘From:’, ‘Reply-To:’ and ‘Sender:’ by typing each one in turn
               and allowing the user to edit the field.  The default values for these fields originate from
               the from, reply-to and sender variables.

     ~h        Edit the message header fields ‘To:’, ‘Cc:’, ‘Bcc:’ and ‘Subject:’ by typing each one in
               turn and allowing the user to edit the field.

     ~I variable
               Insert the value of the specified variable into the message.  The message remains unaltered
               if the variable is unset or empty.  Any embedded character sequences ‘\t’ horizontal tabula‐
               tor and ‘\n’ line feed are expanded in posix mode; otherwise the expansion should occur at
               set time ([v15 behaviour may differ] by using the command modifier wysh).

     ~i variable
               Like ~I, but appends a newline character.

     ~M messages
               Read the named messages into the message being sent, indented by indentprefix.  If no mes‐
               sages are specified, read the current message, the “dot”.

     ~m messages
               Read the named messages into the message being sent, indented by indentprefix.  If no mes‐
               sages are specified, read the current message, the “dot”.  Strips down the list of header
               fields according to the ‘type’ white- and blacklist selection of headerpick.  For MIME mul‐
               tipart messages, only the first displayable part is included.

     ~p        Display the message collected so far, prefaced by the message header fields and followed by
               the attachment list, if any.

     ~Q        Read in the given / current message(s) according to the algorithm of quote.

     ~q        Abort the message being sent, copying it to the file specified by the DEAD variable if save
               is set.

     ~R filename
               Identical to ~r, but indent each line that has been read by indentprefix.

     ~r filename [HERE-delimiter]
               Read the named file, object to the usual Filename transformations, into the message; if (the
               expanded) filename is the hyphen-minus ‘-’ then standard input is used, e.g., for pasting
               purposes.  Only in this latter mode HERE-delimiter may be given: if it is data will be read
               in until the given HERE-delimiter is seen on a line by itself, and encountering EOF is an
               error; the HERE-delimiter is a required argument in non-interactive mode; if it is single-
               quote quoted then the pasted content will not be expanded, [v15 behaviour may differ] other‐
               wise a future version of S-nail may perform shell-style expansion on the content.

     ~s string
               Cause the named string to become the current subject field.  Newline (NL) and carriage-re‐
               turn (CR) bytes are invalid and will be normalized to space (SP) characters.

     ~t name ...
               Add the given name(s) to the direct recipient list.

     ~U messages
               Read in the given / current message(s) excluding all headers, indented by indentprefix.

     ~u messages
               Read in the given / current message(s), excluding all headers.

     ~v        Invoke the VISUAL editor on the message collected so far, then return to compose mode.  ~e
               can be used for a less display oriented editor, and ~|| offers a pipe-based editing ap‐
               proach.

     ~w filename
               Write the message onto the named file, which is object to the usual Filename
               transformations.  If the file exists, the message is appended to it.

     ~x        Same as ~q, except that the message is not saved at all.

INTERNAL VARIABLES
     Internal S-nail variables are controlled via the set and unset commands; prefixing a variable name
     with the string ‘no’ and calling set has the same effect as using unset: ‘unset crt’ and ‘set nocrt’
     do the same thing.  varshow will give more insight on the given variable(s), and set, when called
     without arguments, will show a listing of all variables.  Both commands support a more verbose listing
     mode.  Some well-known variables will also become inherited from the program ENVIRONMENT implicitly,
     others can be imported explicitly with the command environ and henceforth share said properties.

     Two different kinds of internal variables exist, and both of which can also form chains.  There are
     boolean variables, which can only be in one of the two states “set” and “unset”, and value variables
     with a(n optional) string value.  For the latter proper quoting is necessary upon assignment time, the
     introduction of the section COMMANDS documents the supported quoting rules.

           ? wysh set one=val\ 1 two="val 2" \
               three='val "3"' four=$'val \'4\''; \
               varshow one two three four; \
               unset one two three four

     Dependent upon the actual option string values may become interpreted as colour names, command speci‐
     fications, normal text, etc.  They may be treated as numbers, in which case decimal values are ex‐
     pected if so documented, but otherwise any numeric format and base that is valid and understood by the
     vexpr command may be used, too.

     There also exists a special kind of string value, the “boolean string”, which must either be a decimal
     integer (in which case ‘0’ is false and ‘1’ and any other value is true) or any of the (case-insensi‐
     tive) strings ‘off’, ‘no’, ‘n’ and ‘false’ for a false boolean and ‘on’, ‘yes’, ‘y’ and ‘true’ for a
     true boolean; a special kind of boolean string is the “quadoption”, which is a boolean string that can
     optionally be prefixed with the (case-insensitive) term ‘ask-’, as in ‘ask-yes’, which causes prompt‐
     ing of the user in interactive mode, with the given boolean as the default value.

     Variable chains extend a plain ‘variable’ with ‘variable-HOST’ and ‘variable-USER@HOST’ variants.
     Here ‘HOST’ will be converted to all lowercase when looked up (but not when the variable is set or un‐
     set!), [Option]ally IDNA converted, and indeed means ‘server:port’ if a ‘port’ had been specified in
     the contextual Uniform Resource Locator URL, see On URL syntax and credential lookup.  Even though
     this mechanism is based on URLs no URL percent encoding may be applied to neither of ‘USER’ nor
     ‘HOST’, variable chains need to be specified using raw data; the mentioned section contains examples.
     Variables which support chains are explicitly documented as such, and S-nail treats the base name of
     any such variable special, meaning that users should not create custom names like ‘variable-xyz’ in
     order to avoid false classifications and treatment of such variables.

   Initial settings
     The standard POSIX 2008/Cor 2-2016 mandates the following initial variable settings: noallnet,
     noappend, asksub, noaskbcc, noautoprint, nobang, nocmd, nocrt, nodebug, nodot, escape set to ‘~’,
     noflipr, nofolder, header, nohold, noignore, noignoreeof, nokeep, nokeepsave, nometoo, nooutfolder,
     nopage, prompt set to ‘? ’, noquiet, norecord, save, nosendwait, noshowto, noSign, nosign, toplines
     set to ‘5’.

     Notes: S-nail does not support the noonehop variable – use command line options or mta-arguments to
     pass options through to a mta.  And the default global /etc/s-nail.rc file, which is loaded unless the
     -: (with according argument) or -n command line options have been used, or the MAILX_NO_SYSTEM_RC en‐
     vironment variable is set (see Resource files) bends those initial settings a bit, e.g., it sets the
     variables hold, keepsave and keep, to name a few, establishes a default headerpick selection etc., and
     should thus be taken into account.

   Variables
     ?         (Read-only) The exit status of the last command, or the return value of the macro called
               last.  This status has a meaning in the state machine: in conjunction with errexit any non-0
               exit status will cause a program exit, and in posix mode any error while loading (any of
               the) resource files will have the same effect.  ignerr, one of the Command modifiers, can be
               used to instruct the state machine to ignore errors.

     !         (Read-only) The current error number (errno(3)), which is set after an error occurred; it is
               also available via ^ERR, and the error name and documentation string can be queried via
               ^ERRNAME and ^ERRDOC.  [v15 behaviour may differ] This machinery is new and the error number
               is only really usable if a command explicitly states that it manages the variable !, for
               others errno will be used in case of errors, or ^ERR-INVAL if that is 0: it thus may or may
               not reflect the real error.  The error number may be set with the command return.

     ^         (Read-only) This is a multiplexer variable which performs dynamic expansion of the requested
               state or condition, of which there are:

               ^ERR, ^ERRDOC, ^ERRNAME
                         The number, documentation, and name of the current errno(3), respectively, which
                         is usually set after an error occurred.  The documentation is an [Option], the
                         name is used if not available.  [v15 behaviour may differ] This machinery is new
                         and is usually reliable only if a command explicitly states that it manages the
                         variable !, which is effectively identical to ^ERR.  Each of those variables can
                         be suffixed with a hyphen minus followed by a name or number, in which case the
                         expansion refers to the given error.  Note this is a direct mapping of (a subset
                         of) the system error values:

                               define work {
                                 eval echo \$1: \$^ERR-$1:\
                                   \$^ERRNAME-$1: \$^ERRDOC-$1
                                 vput vexpr i + "$1" 1
                                 if [ $i -lt 16 ]
                                   \xcall work $i
                                 end
                               }
                               call work 0

     *         (Read-only) Expands all positional parameters (see 1), separated by the first character of
               the value of ifs.  [v15 behaviour may differ] The special semantics of the equally named
               special parameter of the sh(1) are not yet supported.

     @         (Read-only) Expands all positional parameters (see 1), separated by a space character.  If
               placed in double quotation marks, each positional parameter is properly quoted to expand to
               a single parameter again.

     #         (Read-only) Expands to the number of positional parameters, i.e., the size of the positional
               parameter stack in decimal.

     0         (Read-only) Inside the scope of a defined and called macro this expands to the name of the
               calling macro, or to the empty string if the macro is running from top-level.  For the [Op‐
               tion]al regular expression search and replace operator of vexpr this expands to the entire
               matching expression.  It represents the program name in global context.

     1         (Read-only) Access of the positional parameter stack.  All further parameters can be ac‐
               cessed with this syntax, too, e.g., ‘2’, ‘3’ etc.; positional parameters can be shifted off
               the stack by calling shift.  The parameter stack contains, e.g., the arguments of a called
               defined macro, the matching groups of the [Option]al regular expression search and replace
               expression of vexpr, and can be explicitly created or overwritten with the command vpospar.

     account   (Read-only) Is set to the active account.

     add-file-recipients
               (Boolean) When file or pipe recipients have been specified, mention them in the correspond‐
               ing address fields of the message instead of silently stripping them from their recipient
               list.  By default such addressees are not mentioned.

     allnet    (Boolean) Causes only the local part to be evaluated when comparing addresses.

     append    (Boolean) Causes messages saved in the secondary mailbox MBOX to be appended to the end
               rather than prepended.  This should always be set.

     askatend  (Boolean) Causes the prompts for ‘Cc:’ and ‘Bcc:’ lists to appear after the message has been
               edited.

     askattach
               (Boolean) If set, S-nail asks for files to attach at the end of each message.  An empty line
               finalizes the list.

     askcc     (Boolean) Causes the user to be prompted for carbon copy recipients (at the end of each mes‐
               sage if askatend or bsdcompat are set).

     askbcc    (Boolean) Causes the user to be prompted for blind carbon copy recipients (at the end of
               each message if askatend or bsdcompat are set).

     asksend   (Boolean) Causes the user to be prompted for confirmation to send the message or reenter
               compose mode after having been shown an envelope summary.  This is by default enabled.

     asksign   (Boolean)[Option] Causes the user to be prompted if the message is to be signed at the end
               of each message.  The smime-sign variable is ignored when this variable is set.

     asksub    (Boolean) Causes S-nail to prompt for the subject upon entering compose mode unless a sub‐
               ject already exists.

     attrlist  A sequence of characters to display in the ‘attribute’ column of the headline as shown in
               the display of headers; each for one type of messages (see Message states), with the default
               being ‘NUROSPMFAT+-$~’ or ‘NU  *HMFAT+-$~’ if the bsdflags variable is set, in the following
               order:

               ‘N’       new.
               ‘U’       unread but old.
               ‘R’       new but read.
               ‘O’       read and old.
               ‘S’       saved.
               ‘P’       preserved.
               ‘M’       mboxed.
               ‘F’       flagged.
               ‘A’       answered.
               ‘T’       draft.
               ‘+’       [v15 behaviour may differ] start of a (collapsed) thread in threaded mode (see
                         autosort, thread);
               ‘-’       [v15 behaviour may differ] an uncollapsed thread in threaded mode; only used in
                         conjunction with -L.
               ‘$’       classified as spam.
               ‘~’       classified as possible spam.

     autobcc   Specifies a list of recipients to which a blind carbon copy of each outgoing message will be
               sent automatically.

     autocc    Specifies a list of recipients to which a carbon copy of each outgoing message will be sent
               automatically.

     autocollapse
               (Boolean) Causes threads to be collapsed automatically when .Ql thread Ns ed sort mode is
               entered (see the collapse command).

     autoprint
               (Boolean) Enable automatic typeing of a(n existing) “successive” message after delete and
               undelete commands, e.g., the message that becomes the new “dot” is shown automatically, as
               via dp or dt.

     autosort  Causes sorted mode (see the sort command) to be entered automatically with the value of this
               variable as sorting method when a folder is opened, e.g., ‘set autosort=thread’.

     bang      (Boolean) Enables the substitution of all not (reverse-solidus) escaped exclamation mark ‘!’
               characters by the contents of the last executed command for the ! shell escape command and
               ~!, one of the compose mode COMMAND ESCAPES.  If this variable is not set no reverse solidus
               stripping is performed.

     bind-timeout
               [Option] Terminals generate multi-byte sequences for certain forms of input, for example for
               function and other special keys.  Some terminals however do not write these multi-byte se‐
               quences as a whole, but byte-by-byte, and the latter is what S-nail actually reads.  This
               variable specifies the timeout in milliseconds that the MLE (see On terminal control and
               line editor) waits for more bytes to arrive unless it considers a sequence “complete”.  The
               default is 200.

     bsdcompat
               (Boolean) Sets some cosmetical features to traditional BSD style; has the same affect as
               setting askatend and all other variables prefixed with ‘bsd’; it also changes the behaviour
               of emptystart (which does not exist in BSD).

     bsdflags  (Boolean) Changes the letters shown in the first column of a header summary to traditional
               BSD style.

     bsdheadline
               (Boolean) Changes the display of columns in a header summary to traditional BSD style.

     bsdmsgs   (Boolean) Changes some informational messages to traditional BSD style.

     bsdorder  (Boolean) Causes the ‘Subject:’ field to appear immediately after the ‘To:’ field in message
               headers and with the ~h COMMAND ESCAPES.

     build-cc, build-ld, build-os, build-rest
               (Read-only) The build environment, including the compiler, the linker, the operating system
               S-nail has been build for, usually taken from uname(1) via ‘uname -s’, and then lowercased,
               as well as all the rest that may possibly be useful to include in a bug report, respec‐
               tively.

     charset-7bit
               The value that should appear in the ‘charset=’ parameter of ‘Content-Type:’ MIME header
               fields when no character set conversion of the message data was performed.  This defaults to
               US-ASCII, and the chosen character set should be US-ASCII compatible.

     charset-8bit
               [Option] The default 8-bit character set that is used as an implicit last member of the
               variable sendcharsets.  This defaults to UTF-8 if character set conversion capabilities are
               available, and to ISO-8859-1 otherwise (unless the operating system environment is known to
               always and exclusively support UTF-8 locales), in which case the only supported character
               set is ttycharset and this variable is effectively ignored.  Refer to the section Character
               sets for the complete picture of character set conversion in S-nail.

     charset-unknown-8bit
               [Option] RFC 1428 specifies conditions when internet mail gateways shall “upgrade” the con‐
               tent of a mail message by using a character set with the name ‘unknown-8bit’.  Because of
               the unclassified nature of this character set S-nail will not be capable to convert this
               character set to any other character set.  If this variable is set any message part which
               uses the character set ‘unknown-8bit’ is assumed to really be in the character set given in
               the value, otherwise the (final) value of charset-8bit is used for this purpose.

               This variable will also be taken into account if a MIME type (see The mime.types files) of a
               MIME message part that uses the ‘binary’ character set is forcefully treated as text.

     cmd       The default value for the pipe command.

     colour-disable
               (Boolean)[Option] Forcefully disable usage of colours.  Also see the section Coloured
               display.

     colour-pager
               (Boolean)[Option] Whether colour shall be used for output that is paged through PAGER.  Note
               that pagers may need special command line options, e.g., less(1) requires the option -R and
               lv(1) the option -c in order to support colours.  Often doing manual adjustments is unneces‐
               sary since S-nail may perform adjustments dependent on the value of the environment variable
               PAGER (see there for more).

     contact-mail, contact-web
               (Read-only) Addresses for contact per email and web, respectively, e.g., for bug reports,
               suggestions, or help regarding S-nail.  The former can be used directly: ‘? eval mail
               $contact-mail’.

     crt       In a(n interactive) terminal session, then if this valued variable is set it will be used as
               a threshold to determine how many lines the given output has to span before it will be dis‐
               played via the configured PAGER; Usage of the PAGER can be forced by setting this to the
               value ‘0’, setting it without a value will deduce the current height of the terminal screen
               to compute the threshold (see LINES, screen and stty(1)).  [v15 behaviour may differ] At the
               moment this uses the count of lines of the message in wire format, which, dependent on the
               mime-encoding of the message, is unrelated to the number of display lines.  (The software is
               old and historically the relation was a given thing.)

     customhdr
               Define a set of custom headers to be injected into newly composed or forwarded messages.  A
               custom header consists of the field name followed by a colon ‘:’ and the field content body.
               Standard header field names cannot be overwritten by a custom header.  Different to the com‐
               mand line option -C the variable value is interpreted as a comma-separated list of custom
               headers: to include commas in header bodies they need to become escaped with reverse solidus
               ‘\’.  Headers can be managed more freely in compose mode via ~^.

                     ? set customhdr='Hdr1: Body1-1\, Body1-2, Hdr2: Body2'

     datefield
               Controls the appearance of the ‘%d’ date and time format specification of the headline vari‐
               able, that is used, for example, when viewing the summary of headers.  If unset, then the
               local receiving date is used and displayed unformatted, otherwise the message sending
               ‘Date:’.  It is possible to assign a strftime(3) format string and control formatting, but
               embedding newlines via the ‘%n’ format is not supported, and will result in display errors.
               The default is ‘%Y-%m-%d %H:%M’, and also see datefield-markout-older.

     datefield-markout-older
               Only used in conjunction with datefield.  Can be used to create a visible distinction of
               messages dated more than a day in the future, or older than six months, a concept comparable
               to the -l option of the POSIX utility ls(1).  If set to the empty string, then the plain
               month, day and year of the ‘Date:’ will be displayed, but a strftime(3) format string to
               control formatting can be assigned.  The default is ‘%Y-%m-%d’.

     debug     (Boolean) Enables debug messages and obsoletion warnings, disables the actual delivery of
               messages and also implies norecord as well as nosave.

     disposition-notification-send
               (Boolean)[Option] Emit a ‘Disposition-Notification-To:’ header (RFC 3798) with the message.
               This requires the from variable to be set.

     dot       (Boolean) When dot is set, a period ‘.’ on a line by itself during message input in (inter‐
               active or batch -#) compose mode will be treated as end-of-message (in addition to the nor‐
               mal end-of-file condition).  This behaviour is implied in posix mode with a set ignoreeof.

     dotlock-disable
               (Boolean)[Option] Disable creation of dotlock files for MBOX databases.

     dotlock-ignore-error
               [Obsolete](Boolean)[Option] Ignore failures when creating dotlock files.  Please use
               dotlock-disable instead.

     editalong
               If this variable is set then the editor is started automatically when a message is composed
               in interactive mode.  If the value starts with the letter ‘v’ then this acts as if ~v, oth‐
               erwise as if ~e (see COMMAND ESCAPES) had been specified.  The editheaders variable is im‐
               plied for this automatically spawned editor session.

     editheaders
               (Boolean) When a message is edited while being composed, its header is included in the ed‐
               itable text.

     emptystart
               (Boolean) When entering interactive mode S-nail normally writes “No mail for user” and exits
               immediately if a mailbox is empty or does not exist.  If this variable is set S-nail starts
               even with an empty or non-existent mailbox (the latter behaviour furtherly depends upon
               bsdcompat, though).

     errexit   (Boolean) Let each command with a non-0 exit status, including every called macro which
               returns a non-0 status, cause a program exit unless prefixed by ignerr (see Command
               modifiers).  This also affects COMMAND ESCAPES, but which use a different modifier for ig‐
               noring the error.  Please refer to the variable ? for more on this topic.

     escape    The first character of this value defines the escape character for COMMAND ESCAPES in com‐
               pose mode.  The default value is the character tilde ‘~’.  If set to the empty string, com‐
               mand escapes are disabled.

     expandaddr
               If unset then file and command pipeline address targets are not allowed, and any such ad‐
               dress will be filtered out, giving a warning message.  If set then all possible recipient
               address specifications will be accepted unless a possible value content is more specific
               (also see On sending mail, and non-interactive mode); if desired so only in interactive
               mode, or when tilde commands were enabled explicitly via -~ or -#, the (case-insensitive)
               value ‘restrict’ can be used (this really acts like ‘restrict,-all,+name,+addr’, so that
               care for ordering issues must be taken).

               The value is actually interpreted as a comma-separated list.  If that contains ‘fail’ the
               existence of disallowed addressees is treated as a hard send error instead of only causing
               them to be filtered out.  Address targets can be added and subtracted by prefixing with a
               plus sign ‘+’ or hyphen-minus ‘-’ prefix: the value ‘all’ addresses all possible address
               specifications, ‘file’ file targets, ‘pipe’ command pipeline targets, ‘name’ plain user
               names and (MTA) aliases and ‘addr’ network addresses; Targets are interpreted in the given
               order, so that ‘restrict,fail,+file,-all,+addr’ will cause hard errors for any non-network
               address recipient address unless S-nail is in interactive mode or has been started with the
               -~ or -# command line option; in the latter case(s) any address may be used, then.

               Historically invalid network addressees were silently stripped off.  To change this so that
               any encountered invalid email address causes a hard error it must be ensured that
               ‘failinvaddr’ is an entry in the above list, which automatically enables network addressees;
               it really acts like ‘failinvaddr,+addr’, so that care for ordering issues must be taken.
               [v15 behaviour may differ] If the value ‘shquote’ is present a few address providers (for
               example -b, -c and all recipients given on the command line) will be will evaluated as if
               specified within dollar-single-quotes (see Shell-style argument quoting).

     expandargv
               Unless this variable is set additional mta (Mail-Transfer-Agent) arguments from the command
               line, as can be given after a -- separator, results in a program termination with failure
               status.  The same can be accomplished by using the special (case-insensitive) value ‘fail’.
               A lesser strict variant is the otherwise identical ‘restrict’, which does accept such argu‐
               ments in interactive mode, or if tilde commands were enabled explicitly by using one of the
               command line options -~ or -#.  The empty value will allow unconditional usage.

     features  (Read-only) String giving a list of optional features.  Features are preceded with a plus
               sign ‘+’ if they are available, with a hyphen-minus ‘-’ otherwise.  The output of the com‐
               mand version will include this information in a more pleasant output.

     flipr     (Boolean) This setting reverses the meanings of a set of reply commands, turning the lower‐
               case variants, which by default address all recipients included in the header of a message
               (reply, respond, followup) into the uppercase variants, which by default address the sender
               only (Reply, Respond, Followup) and vice versa.  The commands replysender, respondsender,
               followupsender as well as replyall, respondall, followupall are not affected by the current
               setting of flipr.

     folder    The default path under which mailboxes are to be saved: filenames that begin with the plus
               sign ‘+’ will have the plus sign replaced with the value of this variable if set, otherwise
               the plus sign will remain unchanged when doing Filename transformations; also see file for
               more on this topic, and know about standard imposed implications of outfolder.  The value
               supports a subset of transformations itself, and if the non-empty value does not start with
               a solidus ‘/’, then the value of HOME will be prefixed automatically.  Once the actual value
               is evaluated first, the internal variable folder-resolved will be updated for caching pur‐
               poses.

     folder-hook-FOLDER, folder-hook
               Names a defined macro which will be called whenever a file is opened.  The macro will also
               be invoked when new mail arrives, but message lists for commands executed from the macro
               only include newly arrived messages then.  localopts are activated by default in a folder
               hook, causing the covered settings to be reverted once the folder is left again.

               The specialized form will override the generic one if ‘FOLDER’ matches the file that is
               opened.  Unlike other folder specifications, the fully expanded name of a folder, without
               metacharacters, is used to avoid ambiguities.  However, if the mailbox resides under folder
               then the usual ‘+’ specification is tried in addition, e.g., if folder is “mail” (and thus
               relative to the user's home directory) then /home/usr1/mail/sent will be tried as
               ‘folder-hook-/home/usr1/mail/sent’ first, but then followed by ‘folder-hook-+sent’.

     folder-resolved
               (Read-only) Set to the fully resolved path of folder once that evaluation has occurred;
               rather internal.

     followup-to
               (Boolean) Controls whether a ‘Mail-Followup-To:’ header is generated when sending messages
               to known mailing lists.  Also see followup-to-honour and the commands mlist, mlsubscribe,
               reply and Lreply.

     followup-to-honour
               Controls whether a ‘Mail-Followup-To:’ header is honoured when group-replying to a message
               via reply or Lreply.  This is a quadoption; if set without a value it defaults to “yes”.
               Also see followup-to and the commands mlist and mlsubscribe.

     forward-as-attachment
               (Boolean) Original messages are normally sent as inline text with the forward command, and
               only the first part of a multipart message is included.  With this setting enabled messages
               are sent as unmodified MIME ‘message/rfc822’ attachments with all of their parts included.

     forward-inject-head, forward-inject-tail
               The strings to put before and after the text of a message with the forward command, respec‐
               tively.  The former defaults to ‘-------- Original Message --------\n’.  Special format di‐
               rectives in these strings will be expanded if possible, and if so configured the output will
               be folded according to quote-fold; for more please refer to quote-inject-head.  These vari‐
               ables are ignored if the forward-as-attachment variable is set.

     from      The address (or a list of addresses) to put into the ‘From:’ field of the message header,
               quoting RFC 5322: the author(s) of the message, that is, the mailbox(es) of the person(s) or
               system(s) responsible for the writing of the message.  According to that RFC setting the
               sender variable is required if from contains more than one address.  Dependent on the con‐
               text these addresses are handled as if they were in the list of alternates.

               If a file-based MTA is used, then from (or, if that contains multiple addresses, sender) can
               nonetheless be enforced to appear as the envelope sender address at the MTA protocol level
               (the RFC 5321 reverse-path), either by using the -r command line option (with an empty argu‐
               ment; see there for the complete picture on this topic), or by setting the internal variable
               r-option-implicit.

               If the machine's hostname is not valid at the Internet (for example at a dialup machine)
               then either this variable or hostname ([v15-compat] a SMTP-based mta adds even more fine-
               tuning capabilities with smtp-hostname) have to be set: if so the message and MIME part re‐
               lated unique ID fields ‘Message-ID:’ and ‘Content-ID:’ will be created (except when disal‐
               lowed by message-id-disable or stealthmua).

     fullnames
               (Boolean) Due to historical reasons comments and name parts of email addresses are removed
               by default when sending mail, replying to or forwarding a message.  If this variable is set
               such stripping is not performed.

     fwdheading
               [Obsolete] Predecessor of forward-inject-head.

     header    (Boolean) Causes the header summary to be written at startup and after commands that affect
               the number of messages or the order of messages in the current folder.  Unless in posix mode
               a header summary will also be displayed on folder changes.  The command line option -N can
               be used to set noheader.

     headline  A format string to use for the summary of headers.  Format specifiers in the given string
               start with a percent sign ‘%’ and may be followed by an optional decimal number indicating
               the field width — if that is negative, the field is to be left-aligned.  Names and addresses
               are subject to modifications according to showname and showto.  Valid format specifiers are:

               ‘%%’      A plain percent sign.
               ‘%>’      “Dotmark”: a space character but for the current message (“dot”), for which it ex‐
                         pands to ‘>’ (dependent on headline-plain).
               ‘%<’      “Dotmark”: a space character but for the current message (“dot”), for which it ex‐
                         pands to ‘<’ (dependent on headline-plain).
               ‘%$’      [Option] The spam score of the message, as has been classified via the command
                         spamrate.  Shows only a replacement character if there is no spam support.
               ‘%a’      Message attribute character (status flag); the actual content can be adjusted by
                         setting attrlist.
               ‘%d’      The date found in the ‘Date:’ header of the message when datefield is set (the de‐
                         fault), otherwise the date when the message was received.  Formatting can be con‐
                         trolled by assigning a strftime(3) format string to datefield (and
                         datefield-markout-older).
               ‘%e’      The indenting level in ‘thread’ed sort mode.
               ‘%f’      The address of the message sender.
               ‘%i’      The message thread tree structure.  (Note that this format does not support a
                         field width, and honours headline-plain.)
               ‘%l’      The number of lines of the message, if available.
               ‘%m’      Message number.
               ‘%o’      The number of octets (bytes) in the message, if available.
               ‘%s’      Message subject (if any).
               ‘%S’      Message subject (if any) in double quotes.
               ‘%T’      Message recipient flags: is the addressee of the message a known or subscribed
                         mailing list – see mlist and mlsubscribe.
               ‘%t’      The position in threaded/sorted order.
               ‘U’       The value 0 except in an IMAP mailbox, where it expands to the UID of the message.

               The default is ‘%>%a%m %-18f %16d %4l/%-5o %i%-s’, or ‘%>%a%m %20-f  %16d %3l/%-5o %i%-S’ if
               bsdcompat is set.  Also see attrlist, headline-plain and headline-bidi.

     headline-bidi
               Bidirectional text requires special treatment when displaying headers, because numbers (in
               dates or for file sizes etc.) will not affect the current text direction, in effect result‐
               ing in ugly line layouts when arabic or other right-to-left text is to be displayed.  On the
               other hand only a minority of terminals is capable to correctly handle direction changes, so
               that user interaction is necessary for acceptable results.  Note that extended host system
               support is required nonetheless, e.g., detection of the terminal character set is one pre‐
               condition; and this feature only works in an Unicode (i.e., UTF-8) locale.

               In general setting this variable will cause S-nail to encapsulate text fields that may occur
               when displaying headline (and some other fields, like dynamic expansions in prompt) with
               special Unicode control sequences; it is possible to fine-tune the terminal support level by
               assigning a value: no value (or any value other than ‘1’, ‘2’ and ‘3’) will make S-nail as‐
               sume that the terminal is capable to properly deal with Unicode version 6.3, in which case
               text is embedded in a pair of U+2068 (FIRST STRONG ISOLATE) and U+2069 (POP DIRECTIONAL ISO‐
               LATE) characters.  In addition no space on the line is reserved for these characters.

               Weaker support is chosen by using the value ‘1’ (Unicode 6.3, but reserve the room of two
               spaces for writing the control sequences onto the line).  The values ‘2’ and ‘3’ select Uni‐
               code 1.1 support (U+200E, LEFT-TO-RIGHT MARK); the latter again reserves room for two spaces
               in addition.

     headline-plain
               (Boolean) On Unicode (UTF-8) aware terminals enhanced graphical symbols are used by default
               for certain entries of headline.  If this variable is set only basic US-ASCII symbols will
               be used.

     history-file
               [Option] If a line editor is available then this can be set to name the (expandable) path of
               the location of a permanent history file; also see history-size.

     history-gabby
               (Boolean)[Option] Add more entries to the history as is normally done.

     history-gabby-persist
               (Boolean)[Option] S-nail's own MLE will not save the additional history-gabby entries in
               persistent storage unless this variable is set.  On the other hand it will not loose the
               knowledge of whether a persistent entry was gabby or not.  Also see history-file.

     history-size
               [Option] Setting this variable imposes a limit on the number of concurrent history entries.
               If set to the value 0 then no further history entries will be added, and loading and incor‐
               poration of the history-file upon program startup can also be suppressed by doing this.
               Runtime changes will not be reflected before the history is saved or loaded (again).

     hold      (Boolean) This setting controls whether messages are held in the system inbox, and it is set
               by default.

     hostname  Used instead of the value obtained from uname(3) and getaddrinfo(3) as the hostname when ex‐
               panding local addresses, e.g., in ‘From:’ (also see On sending mail, and non-interactive
               mode, especially for expansion of network addresses that contain domain-less valid user
               names in angle brackets).  If either of from or this variable Is set the message and MIME
               part related unique ID fields ‘Message-ID:’ and ‘Content-ID:’ will be created (except when
               disallowed by message-id-disable or stealthmua).  If the [Option]al IDNA support is avail‐
               able (see idna-disable) variable assignment is aborted when a necessary conversion fails.

               Setting it to the empty string will cause the normal hostname to be used, but nonetheless
               enables creation of said ID fields.  [v15-compat] in conjunction with the built-in SMTP mta
               smtp-hostname also influences the results: one should produce some test messages with the
               desired combination of hostname, and/or from, sender etc. first.

     idna-disable
               (Boolean)[Option] Can be used to turn off the automatic conversion of domain names according
               to the rules of IDNA (internationalized domain names for applications).  Since the IDNA code
               assumes that domain names are specified with the ttycharset character set, an UTF-8 locale
               charset is required to represent all possible international domain names (before conversion,
               that is).

     ifs       The input field separator that is used ([v15 behaviour may differ] by some functions) to de‐
               termine where to split input data.

               1.        Unsetting is treated as assigning the default value, ‘ \t\n’.
               2.        If set to the empty value, no field splitting will be performed.
               3.        If set to a non-empty value, all whitespace characters are extracted and assigned
                         to the variable ifs-ws.

               a.        ifs-ws will be ignored at the beginning and end of input.  Diverging from POSIX
                         shells default whitespace is removed in addition, which is owed to the entirely
                         different line content extraction rules.
               b.        Each occurrence of a character of ifs will cause field-splitting, any adjacent
                         ifs-ws characters will be skipped.

     ifs-ws    (Read-only) Automatically deduced from the whitespace characters in ifs.

     ignore    (Boolean) Ignore interrupt signals from the terminal while entering messages; instead echo
               them as ‘@’ characters and discard the current line.

     ignoreeof
               (Boolean) Ignore end-of-file conditions (‘control-D’) in compose mode on message input and
               in interactive command input.  If set an interactive command input session can only be left
               by explicitly using one of the commands exit and quit, and message input in compose mode can
               only be terminated by entering a period ‘.’ on a line by itself or by using the ~. COMMAND
               ESCAPES; Setting this implies the behaviour that dot describes in posix mode.

     inbox     If this is set to a non-empty string it will specify the user's primary system mailbox,
               overriding MAIL and the system-dependent default, and (thus) be used to replace ‘%’ when do‐
               ing Filename transformations; also see file for more on this topic.  The value supports a
               subset of transformations itself.

     indentprefix
               String used by the ~m, ~M and ~R COMMAND ESCAPES and by the quote option for indenting mes‐
               sages, in place of the POSIX mandated default tabulator character ‘\t’.  Also see
               quote-chars.

     keep      (Boolean) If set, an empty primary system mailbox file is not removed.  Note that, in con‐
               junction with posix mode any empty file will be removed unless this variable is set.  This
               may improve the interoperability with other mail user agents when using a common folder di‐
               rectory, and prevents malicious users from creating fake mailboxes in a world-writable spool
               directory.  [v15 behaviour may differ] Only local regular (MBOX) files are covered, Maildir
               and other mailbox types will never be removed, even if empty.

     keep-content-length
               (Boolean) When (editing messages and) writing MBOX mailbox files S-nail can be told to keep
               the ‘Content-Length:’ and ‘Lines:’ header fields that some MUAs generate by setting this
               variable.  Since S-nail does neither use nor update these non-standardized header fields
               (which in itself shows one of their conceptual problems), stripping them should increase in‐
               teroperability in between MUAs that work with with same mailbox files.  Note that, if this
               is not set but writebackedited, as below, is, a possibly performed automatic stripping of
               these header fields already marks the message as being modified.  [v15 behaviour may differ]
               At some future time S-nail will be capable to rewrite and apply an mime-encoding to modified
               messages, and then those fields will be stripped silently.

     keepsave  (Boolean) When a message is saved it is usually discarded from the originating folder when
               S-nail is quit.  This setting causes all saved message to be retained.

     line-editor-disable
               (Boolean) Turn off any enhanced line editing capabilities (see On terminal control and line
               editor for more).

     line-editor-no-defaults
               (Boolean)[Option] Do not establish any default key binding.

     log-prefix
               Error log message prefix string (‘s-nail: ’).

     mailbox-display
               (Read-only) The name of the current mailbox (file), possibly abbreviated for display pur‐
               poses.

     mailbox-resolved
               (Read-only) The fully resolved path of the current mailbox.

     mailx-extra-rc
               An additional startup file that is loaded as the last of the Resource files.  Use this file
               for commands that are not understood by other POSIX mailx(1) implementations, i.e., mostly
               anything which is not covered by Initial settings.

     markanswered
               (Boolean) When a message is replied to and this variable is set, it is marked as having been
               answered.  See the section Message states.

     mbox-rfc4155
               (Boolean) When opening MBOX mailbox databases S-nail by default uses tolerant POSIX rules
               for detecting message boundaries (so-called ‘From_’ lines) due to compatibility reasons, in‐
               stead of the stricter rules that have been standardized in RFC 4155.  This behaviour can be
               switched to the stricter RFC 4155 rules by setting this variable.  (This is never necessary
               for any message newly generated by S-nail, it only applies to messages generated by buggy or
               malicious MUAs, or may occur in old MBOX databases: S-nail itself will choose a proper
               mime-encoding to avoid false interpretation of ‘From_’ content lines in the MBOX database.)

               This may temporarily be handy when S-nail complains about invalid ‘From_’ lines when opening
               a MBOX: in this case setting this variable and re-opening the mailbox in question may cor‐
               rect the result.  If so, copying the entire mailbox to some other file, as in ‘copy *
               SOME-FILE’, will perform proper, all-compatible ‘From_’ quoting for all detected messages,
               resulting in a valid MBOX mailbox.  Finally the variable can be unset again:

                     define mboxfix {
                       localopts yes; wysh set mbox-rfc4155;\
                         wysh File "${1}"; eval copy * "${2}"
                     }
                     call mboxfix /tmp/bad.mbox /tmp/good.mbox

     memdebug  (Boolean) Internal development variable.

     message-id-disable
               (Boolean) By setting this variable the generation of ‘Message-ID:’ and ‘Content-ID:’ message
               and MIME part headers can be completely suppressed, effectively leaving this task up to the
               mta (Mail-Transfer-Agent) or the SMTP server.  Note that according to RFC 5321 a SMTP server
               is not required to add this field by itself, so it should be ensured that it accepts mes‐
               sages without ‘Message-ID’.

     message-inject-head
               A string to put at the beginning of each new message, followed by a newline.  [Obsolete] The
               escape sequences tabulator ‘\t’ and newline ‘\n’ are understood (use the wysh prefix when
               setting the variable(s) instead).

     message-inject-tail
               A string to put at the end of each new message, followed by a newline.  [Obsolete] The es‐
               cape sequences tabulator ‘\t’ and newline ‘\n’ are understood (use the wysh prefix when
               setting the variable(s) instead).

     metoo     (Boolean) Usually, when an alias expansion contains the sender, the sender is removed from
               the expansion.  Setting this option suppresses these removals.  Note that a set metoo also
               causes a ‘-m’ option to be passed through to the mta (Mail-Transfer-Agent); though most of
               the modern MTAs no longer document this flag, no MTA is known which does not support it (for
               historical compatibility).

     mime-allow-text-controls
               (Boolean) When sending messages, each part of the message is MIME-inspected in order to
               classify the ‘Content-Type:’ and ‘Content-Transfer-Encoding:’ (see mime-encoding) that is
               required to send this part over mail transport, i.e., a computation rather similar to what
               the file(1) command produces when used with the ‘--mime’ option.

               This classification however treats text files which are encoded in UTF-16 (seen for HTML
               files) and similar character sets as binary octet-streams, forcefully changing any
               ‘text/plain’ or ‘text/html’ specification to ‘application/octet-stream’: If that actually
               happens a yet unset charset MIME parameter is set to ‘binary’, effectively making it impos‐
               sible for the receiving MUA to automatically interpret the contents of the part.

               If this variable is set, and the data was unambiguously identified as text data at first
               glance (by a ‘.txt’ or ‘.html’ file extension), then the original ‘Content-Type:’ will not
               be overwritten.

     mime-alternative-favour-rich
               (Boolean) If this variable is set then rich MIME alternative parts (e.g., HTML) will be pre‐
               ferred in favour of included plain text versions when displaying messages, provided that a
               handler exists which produces output that can be (re)integrated into S-nail's normal visual
               display.  (E.g., at the time of this writing some newsletters ship their full content only
               in the rich HTML part, whereas the plain text part only contains topic subjects.)

     mime-counter-evidence
               Normally the ‘Content-Type:’ field is used to decide how to handle MIME parts.  Some MUAs,
               however, do not use The mime.types files (also see HTML mail and MIME attachments) or a sim‐
               ilar mechanism to correctly classify content, but specify an unspecific MIME type
               (‘application/octet-stream’) even for plain text attachments.  If this variable is set then
               S-nail will try to re-classify such MIME message parts, if possible, for example via a pos‐
               sibly existing attachment filename.  A non-empty value may also be given, in which case a
               number is expected, actually a carrier of bits, best specified as a binary value, e.g.,
               ‘0b1111’.

               •   If bit two is set (counting from 1, decimal 2) then the detected mimetype will be car‐
                   ried along with the message and be used for deciding which MIME handler is to be used,
                   for example; when displaying such a MIME part the part-info will indicate the overridden
                   content-type by showing a plus sign ‘+’.
               •   If bit three is set (decimal 4) then the counter-evidence is always produced and a posi‐
                   tive result will be used as the MIME type, even forcefully overriding the parts given
                   MIME type.
               •   If bit four is set (decimal 8) as a last resort the actual content of
                   ‘application/octet-stream’ parts will be inspected, so that data which looks like plain
                   text can be treated as such.  This mode is even more relaxed when data is to be dis‐
                   played to the user or used as a message quote (data consumers which mangle data for dis‐
                   play purposes, which includes masking of control characters, for example).

     mime-encoding
               The MIME ‘Content-Transfer-Encoding’ to use in outgoing text messages and message parts,
               where applicable.  (7-bit clean text messages are sent as-is, without a transfer encoding.)
               Valid values are:

               ‘8bit’    (Or ‘8b’.)  8-bit transport effectively causes the raw data be passed through un‐
                         changed, but may cause problems when transferring mail messages over channels that
                         are not ESMTP (RFC 1869) compliant.  Also, several input data constructs are not
                         allowed by the specifications and may cause a different transfer-encoding to be
                         used.
               ‘quoted-printable’
                         (Or ‘qp’.)  Quoted-printable encoding is 7-bit clean and has the property that
                         ASCII characters are passed through unchanged, so that an english message can be
                         read as-is; it is also acceptable for other single-byte locales that share many
                         characters with ASCII, like, e.g., ISO-8859-1.  The encoding will cause a large
                         overhead for messages in other character sets: e.g., it will require up to twelve
                         (12) bytes to encode a single UTF-8 character of four (4) bytes.  It is the de‐
                         fault encoding.
               ‘base64’  (Or ‘b64’.)  This encoding is 7-bit clean and will always be used for binary data.
                         This encoding has a constant input:output ratio of 3:4, regardless of the charac‐
                         ter set of the input data it will encode three bytes of input to four bytes of
                         output.  This transfer-encoding is not human readable without performing a decod‐
                         ing step.

     mimetypes-load-control
               Can be used to control which of The mime.types files are loaded: if the letter ‘u’ is part
               of the option value, then the user's personal ~/.mime.types file will be loaded (if it ex‐
               ists); likewise the letter ‘s’ controls loading of the system wide /etc/mime.types; direc‐
               tives found in the user file take precedence, letter matching is case-insensitive.  If this
               variable is not set S-nail will try to load both files.  Incorporation of the
               S-nail-built-in MIME types cannot be suppressed, but they will be matched last (the order
               can be listed via mimetype).

               More sources can be specified by using a different syntax: if the value string contains an
               equals sign ‘=’ then it is instead parsed as a comma-separated list of the described letters
               plus ‘f=FILENAME’ pairs; the given filenames will be expanded and loaded, and their content
               may use the extended syntax that is described in the section The mime.types files.  Direc‐
               tives found in such files always take precedence (are prepended to the MIME type cache).

     mta       To choose an alternate Mail-Transfer-Agent, set this option to either the full pathname of
               an executable (optionally prefixed with the protocol ‘file://’), or [Option]ally a SMTP
               a.k.a. SUBMISSION protocol URL, e.g., [v15-compat]

                     submissions://[user[:password]@]server[:port]

               ([no v15-compat]: ‘[smtp://]server[:port]’.)  The default has been chosen at compile time.
               All supported data transfers are executed in child processes, which run asynchronously and
               without supervision unless either the sendwait or the verbose variable is set.  If such a
               child receives a TERM signal, it will abort and save the message to DEAD, if so configured.

               For a file-based MTA it may be necessary to set mta-argv0 in in order to choose the right
               target of a modern mailwrapper(8) environment.  It will be passed command line arguments
               from several possible sources: from the variable mta-arguments if set, from the command line
               if given and the variable expandargv allows their use.  Argument processing of the MTA will
               be terminated with a -- separator.

               The otherwise occurring implicit usage of the following MTA command line arguments can be
               disabled by setting the boolean variable mta-no-default-arguments (which will also disable
               passing -- to the MTA): -i (for not treating a line with only a dot ‘.’ character as the end
               of input), -m (shall the variable metoo be set) and -v (if the verbose variable is set); in
               conjunction with the -r command line option S-nail will also (not) pass -f as well as possi‐
               bly -F.

               [Option]ally S-nail can send mail over SMTP a.k.a. SUBMISSION network connections to a sin‐
               gle defined smart host by setting this variable to a SMTP or SUBMISSION URL (see On URL
               syntax and credential lookup).  An authentication scheme can be specified via the variable
               chain smtp-auth.  Encrypted network connections are [Option]ally available, the section
               Encrypted network communication should give an overview and provide links to more informa‐
               tion on this.  Note that with some mail providers it may be necessary to set the
               smtp-hostname variable in order to use a specific combination of from, hostname and mta.
               S-nail also supports forwarding of all network traffic over a specified socks-proxy.  The
               following SMTP variants may be used:

               •   The plain SMTP protocol (RFC 5321) that normally lives on the server port 25 and re‐
                   quires setting the smtp-use-starttls variable to enter a TLS encrypted session state.
                   Assign a value like [v15-compat] ‘smtp://[user[:password]@]server[:port]’ ([no v15-com‐
                   pat] ‘smtp://server[:port]’) to choose this protocol.

               •   The so-called SMTPS which is supposed to live on server port 465 and is automatically
                   TLS secured.  Unfortunately it never became a standardized protocol and may thus not be
                   supported by your hosts network service database – in fact the port number has already
                   been reassigned to other protocols!

                   SMTPS is nonetheless a commonly offered protocol and thus can be chosen by assigning a
                   value like [v15-compat] ‘smtps://[user[:password]@]server[:port]’ ([no v15-compat]
                   ‘smtps://server[:port]’); due to the mentioned problems it is usually necessary to ex‐
                   plicitly specify the port as ‘:465’, however.

               •   The SUBMISSION protocol (RFC 6409) lives on server port 587 and is identically to the
                   SMTP protocol from S-nail's point of view; it requires setting smtp-use-starttls to en‐
                   ter a TLS secured session state; e.g., [v15-compat]
                   ‘submission://[user[:password]@]server[:port]’.

               •   The SUBMISSIONS protocol (RFC 8314) that lives on server port 465 and is TLS secured by
                   default.  It can be chosen by assigning a value like [v15-compat]
                   ‘submissions://[user[:password]@]server[:port]’.  Due to the problems mentioned for
                   SMTPS above and the fact that SUBMISSIONS is new and a successor that lives on the same
                   port as the historical engineering mismanagement named SMTPS, it is usually necessary to
                   explicitly specify the port as ‘:465’.

     mta-arguments
               Arguments to pass through to a file-based mta can be given via this variable, which is
               parsed according to Shell-style argument quoting into an array of arguments, and which will
               be joined onto MTA options from other sources, and then passed individually to the MTA: ‘?
               wysh set mta-arguments='-t -X "/tmp/my log"'’.

     mta-no-default-arguments
               (Boolean) Unless this variable is set S-nail will pass some well known standard command line
               options to a file-based mta (Mail-Transfer-Agent), see there for more.

     mta-no-receiver-arguments
               (Boolean) By default a file-based mta will be passed all receiver addresses on the command
               line.  This variable can be set to suppress any such argument.

     mta-argv0
               Many systems use a so-called mailwrapper(8) environment to ensure compatibility with
               sendmail(1).  This works by inspecting the name that was used to invoke the mail delivery
               system.  If this variable is set then the mailwrapper (the program that is actually executed
               when calling the file-based mta) will treat its contents as that name.

     netrc-lookup-USER@HOST, netrc-lookup-HOST, netrc-lookup
               (Boolean)[v15-compat][Option] Used to control usage of the user's ~/.netrc file for lookup
               of account credentials, as documented in the section On URL syntax and credential lookup and
               for the command netrc; the section The .netrc file documents the file format.  Also see
               netrc-pipe.

     netrc-pipe
               [v15-compat][Option] When ~/.netrc is loaded (see netrc and netrc-lookup) then S-nail will
               read the output of a shell pipe instead of the user's ~/.netrc file if this variable is set
               (to the desired shell command).  This can be used to, e.g., store ~/.netrc in encrypted
               form: ‘? set netrc-pipe='gpg -qd ~/.netrc.pgp'’.

     newfolders
               [Option] If this variable has the value ‘maildir’, newly created local folders will be in
               Maildir instead of MBOX format.

     newmail   Checks for new mail in the current folder each time the prompt is shown.  A Maildir folder
               must be re-scanned to determine if new mail has arrived.  If this variable is set to the
               special value ‘nopoll’ then a Maildir folder will not be rescanned completely, but only
               timestamp changes are detected.  Maildir folders are [Option]al.

     outfolder
               (Boolean) Unless specified as absolute pathnames, causes the filename given in the record
               variable and the sender-based filenames for the Copy and Save commands to be interpreted
               relative to the directory given in the folder variable rather than relative to the current
               directory.

     on-account-cleanup-ACCOUNT, on-account-cleanup
               Macro hook which will be called once an account is left, as the very last step before un‐
               rolling per-account localopts.  This hook is run even in case of fatal errors, and it is ad‐
               visable to perform only absolutely necessary actions, like cleaning up alternates, for exam‐
               ple.  The specialized form is used in favour of the generic one if found.

     on-compose-cleanup
               Macro hook which will be called after the message has been sent (or not, in case of fail‐
               ures), as the very last step before unrolling compose mode localopts.  This hook is run even
               in case of fatal errors, and it is advisable to perform only absolutely necessary actions,
               like cleaning up alternates, for example.

               For compose mode hooks that may affect the message content please see on-compose-enter,
               on-compose-leave, on-compose-splice.  [v15 behaviour may differ] This hook exists because
               alias, alternates, commandalias, shortcut, to name a few, are not covered by localopts:
               changes applied in compose mode will continue to be in effect thereafter.

     on-compose-enter, on-compose-leave
               Macro hooks which will be called once compose mode is entered, and after composing has been
               finished, but before a set message-inject-tail has been injected etc., respectively.
               localopts are enabled for these hooks, and changes on variables will be forgotten after the
               message has been sent.  on-compose-cleanup can be used to perform other necessary cleanup
               steps.

               The following (read-only) variables will be set temporarily during execution of the macros
               to represent respective message headers, to the empty string otherwise; most of them corre‐
               spond to according virtual message headers that can be accessed via ~^, one of the COMMAND
               ESCAPES (also from within on-compose-splice hooks):

               mailx-command  The command that generates the message.
               mailx-subject  The subject.
               mailx-from     from.
               mailx-sender   sender.
               mailx-to, mailx-cc, mailx-bcc
                              The list of receiver addresses as a space-separated list.
               mailx-raw-to, mailx-raw-cc, mailx-raw-bcc
                              The list of receiver addresses before any mangling (due to, e.g., alternates,
                              alias recipients-in-cc) as a space-separated list.
               mailx-orig-from
                              When replying, forwarding or resending, this will be set to the ‘From:’ of
                              the given message.
               mailx-orig-to, mailx-orig-cc, mailx-orig-bcc
                              When replying, forwarding or resending, this will be set to the receivers of
                              the given message.

               Here is am example that injects a signature via message-inject-tail; instead using
               on-compose-splice to simply inject the file of desire via ~< or ~<! may be a better ap‐
               proach.

                     define t_ocl {
                       vput ! i cat ~/.mysig
                       if [ $? -eq 0 ]
                          vput vexpr message-inject-tail trim-end $i
                       end

                       # Alternatively
                       readctl create ~/.mysig
                       if [ $? -eq 0 ]
                         readall i
                         if [ $? -eq 0 ]
                           vput vexpr message-inject-tail trim-end $i
                         end
                         readctl remove ~/.mysig
                       end
                     }
                     set on-compose-leave=t_ocl

     on-compose-splice, on-compose-splice-shell
               These hooks run once the normal compose mode is finished, but before the on-compose-leave
               macro hook is called, the message-inject-tail is injected etc.  Both hooks will be executed
               in a subprocess, with their input and output connected to S-nail such that they can act as
               if they would be an interactive user.  The difference in between them is that the latter is
               a SHELL command, whereas the former is a normal S-nail macro, but which is restricted to a
               small set of commands (the verbose output of, e.g., list will indicate said capability).
               localopts are enabled for these hooks (in the parent process), causing any setting to be
               forgotten after the message has been sent; on-compose-cleanup can be used to perform other
               cleanup as necessary.

               During execution of these hooks S-nail will temporarily forget whether it has been started
               in interactive mode, (a restricted set of) COMMAND ESCAPES will always be available, and for
               guaranteed reproducibilities sake escape and ifs will be set to their defaults.  The compose
               mode command ~^ has been especially designed for scriptability (via these hooks).  The first
               line the hook will read on its standard input is the protocol version of said command es‐
               cape, currently “0 0 1”: backward incompatible protocol changes have to be expected.

               Care must be taken to avoid deadlocks and other false control flow: if both involved pro‐
               cesses wait for more input to happen at the same time, or one does not expect more input but
               the other is stuck waiting for consumption of its output, etc.  There is no automatic syn‐
               chronization of the hook: it will not be stopped automatically just because it, e.g., emits
               ‘~x’.  The hooks will however receive a termination signal if the parent enters an error
               condition.  [v15 behaviour may differ] Protection against and interaction with signals is
               not yet given; it is likely that in the future these scripts will be placed in an isolated
               session, which is signalled in its entirety as necessary.

                     define ocs_signature {
                       read version
                       echo '~< ~/.mysig' # '~<! fortune pathtofortunefile'
                     }
                     set on-compose-splice=ocs_signature

                     wysh set on-compose-splice-shell=$'\
                       read version;\
                       printf "hello $version!  Headers: ";\
                       echo \'~^header list\';\
                       read status result;\
                       echo "status=$status result=$result";\
                       '

                     define ocsm {
                       read version
                       echo Splice protocol version is $version
                       echo '~^h l'; read hl; vput vexpr es substring "${hl}" 0 1
                       if [ "$es" != 2 ]
                         echoerr 'Cannot read header list'; echo '~x'; xit
                       endif
                       if [ "$hl" @i!% ' cc' ]
                         echo '~^h i cc Diet is your <mirr.or>'; read es;\
                           vput vexpr es substring "${es}" 0 1
                         if [ "$es" != 2 ]
                           echoerr 'Cannot insert Cc: header'; echo '~x'
                           # (no xit, macro finishs anyway)
                         endif
                       endif
                     }
                     set on-compose-splice=ocsm

     on-resend-cleanup
               [v15 behaviour may differ] Identical to on-compose-cleanup, but is only triggered by resend.

     on-resend-enter
               [v15 behaviour may differ] Identical to on-compose-enter, but is only triggered by resend.

     page      (Boolean) If set, each message feed through the command given for pipe is followed by a
               formfeed character ‘\f’.

     password-USER@HOST, password-HOST, password
               [v15-compat] Variable chain that sets a password, which is used in case none has been given
               in the protocol and account-specific URL; as a last resort S-nail will ask for a password on
               the user's terminal if the authentication method requires a password.  Specifying passwords
               in a startup file is generally a security risk; the file should be readable by the invoking
               user only.

     password-USER@HOST
               [no v15-compat] (see the chain above for [v15-compat]) Set the password for ‘USER’ when con‐
               necting to ‘HOST’.  If no such variable is defined for a host, the user will be asked for a
               password on standard input.  Specifying passwords in a startup file is generally a security
               risk; the file should be readable by the invoking user only.

     piperaw   (Boolean) Send messages to the pipe command without performing MIME and character set con‐
               versions.

     pipe-TYPE/SUBTYPE
               When a MIME message part of type ‘TYPE/SUBTYPE’ (case-insensitive) is displayed or quoted,
               its text is filtered through the value of this variable interpreted as a shell command.
               Note that only parts which can be displayed inline as plain text (see copiousoutput) are
               displayed unless otherwise noted, other MIME parts will only be considered by and for the
               command mimeview.

               The special value commercial at ‘@’ forces interpretation of the message part as plain text,
               e.g., ‘set pipe-application/xml=@’ will henceforth display XML “as is”.  (The same could
               also be achieved by adding a MIME type marker with the mimetype command.  And [Option]ally
               MIME type handlers may be defined via The Mailcap files — these directives, copiousoutput
               has already been used, should be referred to for further documentation.

               The commercial at ‘@’ can in fact be used as a trigger character to adjust usage and behav‐
               iour of a following shell command specification more thoroughly by appending more special
               characters which refer to further mailcap directives, e.g., the following hypothetical com‐
               mand specification could be used:

                     ? set pipe-X/Y='@!++=@vim ${MAILX_FILENAME_TEMPORARY}'

               ‘*’       The command produces plain text to be integrated in S-nails output: copiousoutput.
               ‘#’       If set the handler will not be invoked when a message is to be quoted, but only
                         when it will be displayed: x-mailx-noquote.
               ‘&’       Run the command asynchronously, i.e., without blocking S-nail: x-mailx-async.
               ‘!’       The command must be run on an interactive terminal, S-nail will temporarily re‐
                         lease the terminal to it: needsterminal.
               ‘+’       Request creation of a zero-sized temporary file, the absolute pathname of which
                         will be made accessible via the environment variable MAILX_FILENAME_TEMPORARY:
                         x-mailx-tmpfile.  If given twice then the file will be unlinked automatically by
                         S-nail when the command loop is entered again at latest: x-mailx-tmpfile-unlink.
               ‘=’       Normally the MIME part content is passed to the handler via standard input; if
                         this flag is set then the data will instead be written into
                         MAILX_FILENAME_TEMPORARY (x-mailx-tmpfile-fill), the creation of which is implied;
                         note however that in order to cause deletion of the temporary file you still have
                         to use two plus signs ‘++’ explicitly!
               ‘@’       To avoid ambiguities with normal shell command content you can use another commer‐
                         cial at to forcefully terminate interpretation of remaining characters.  (Any
                         character not in this list will have the same effect.)

               Some information about the MIME part to be displayed is embedded into the environment of the
               shell command:

               MAILX_CONTENT            The MIME content-type of the part, if known, the empty string oth‐
                                        erwise.
               MAILX_CONTENT_EVIDENCE   If mime-counter-evidence includes the carry-around-bit (2), then
                                        this will be set to the detected MIME content-type; not only then
                                        identical to MAILX_CONTENT otherwise.
               MAILX_EXTERNAL_BODY_URL  MIME parts of type ‘message/external-body access-type=url’ will
                                        store the access URL in this variable, it is empty otherwise.  URL
                                        targets should not be activated automatically, without supervision.
               MAILX_FILENAME           The filename, if any is set, the empty string otherwise.
               MAILX_FILENAME_GENERATED
                                        A random string.
               MAILX_FILENAME_TEMPORARY
                                        If temporary file creation has been requested through the command
                                        prefix this variable will be set and contain the absolute pathname
                                        of the temporary file.

     pipe-EXTENSION
               This is identical to pipe-TYPE/SUBTYPE except that ‘EXTENSION’ (normalized to lowercase us‐
               ing character mappings of the ASCII charset) names a file extension, e.g., ‘xhtml’.  Han‐
               dlers registered using this method take precedence.

     pop3-auth-USER@HOST, pop3-auth-HOST, pop3-auth
               [Option][v15-compat] Variable chain that sets the POP3 authentication method.  The only pos‐
               sible value as of now is ‘plain’, which is thus the default.

     pop3-bulk-load-USER@HOST, pop3-bulk-load-HOST, pop3-bulk-load
               (Boolean)[Option] When accessing a POP3 server S-nail loads the headers of the messages, and
               only requests the message bodies on user request.  For the POP3 protocol this means that the
               message headers will be downloaded twice.  If this variable is set then S-nail will download
               only complete messages from the given POP3 server(s) instead.

     pop3-keepalive-USER@HOST, pop3-keepalive-HOST, pop3-keepalive
               [Option] POP3 servers close the connection after a period of inactivity; the standard re‐
               quires this to be at least 10 minutes, but practical experience may vary.  Setting this
               variable to a numeric value greater than ‘0’ causes a ‘NOOP’ command to be sent each value
               seconds if no other operation is performed.

     pop3-no-apop-USER@HOST, pop3-no-apop-HOST, pop3-no-apop
               (Boolean)[Option] Unless this variable is set the ‘APOP’ authentication method will be used
               when connecting to a POP3 server that advertises support.  The advantage of ‘APOP’ is that
               the password is not sent in clear text over the wire and that only a single packet is sent
               for the user/password tuple.  Note that pop3-no-apop-HOST requires [v15-compat].

     pop3-use-starttls-USER@HOST, pop3-use-starttls-HOST, pop3-use-starttls
               (Boolean)[Option] Causes S-nail to issue a ‘STLS’ command to make an unencrypted POP3 ses‐
               sion TLS encrypted.  This functionality is not supported by all servers, and is not used if
               the session is already encrypted by the POP3S method.  Note that pop3-use-starttls-HOST re‐
               quires [v15-compat].

     posix     (Boolean) This flag enables POSIX mode, which changes behaviour of S-nail where that devi‐
               ates from standardized behaviour.  It will be set implicitly before the Resource files are
               loaded if the environment variable POSIXLY_CORRECT is set, and adjusting any of those two
               will be reflected by the other one implicitly.  The following behaviour is covered and en‐
               forced by this mechanism:

               •   In non-interactive mode, any error encountered while loading resource files during pro‐
                   gram startup will cause a program exit, whereas in interactive mode such errors will
                   stop loading of the currently loaded (stack of) file(s, i.e., recursively).  These exits
                   can be circumvented on a per-command base by using ignerr, one of the Command modifiers,
                   for each command which shall be allowed to fail.
               •   alternates will replace the list of alternate addresses instead of appending to it.  In
                   addition alternates will only be honoured for any sort of message reply, and for
                   aliases.
               •   The variable inserting COMMAND ESCAPES ~A, ~a, ~I and ~i will expand embedded character
                   sequences ‘\t’ horizontal tabulator and ‘\n’ line feed.  [v15 behaviour may differ] For
                   compatibility reasons this step will always be performed.
               •   Upon changing the active file no summary of headers will be displayed even if header is
                   set.
               •   Setting ignoreeof implies the behaviour described by dot.
               •   The variable keep is extended to cover any empty mailbox, not only empty primary system
                   mailboxes: they will be removed when they are left in empty state otherwise.

     print-alternatives
               (Boolean) When a MIME message part of type ‘multipart/alternative’ is displayed and it con‐
               tains a subpart of type ‘text/plain’, other parts are normally discarded.  Setting this
               variable causes all subparts to be displayed, just as if the surrounding part was of type
               ‘multipart/mixed’.

     prompt    The string used as a prompt in interactive mode.  Whenever the variable is evaluated the
               value is treated as if specified within dollar-single-quotes (see Shell-style argument
               quoting).  This (post-assignment, i.e., second) expansion can be used to embed status infor‐
               mation, for example ?, !, account or mailbox-display.

               In order to embed characters which should not be counted when calculating the visual width
               of the resulting string, enclose the characters of interest in a pair of reverse solidus es‐
               caped brackets: ‘\[\E[0m\]’; a slot for coloured prompts is also available with the [Op‐
               tion]al command colour.  Prompting may be prevented by setting this to the null string
               (a.k.a. ‘set noprompt’).

     prompt2   This string is used for secondary prompts, but is otherwise identical to prompt.  The de‐
               fault is ‘.. ’.

     quiet     (Boolean) Suppresses the printing of the version when first invoked.

     quote     If set a reply message is started with the quoted original message, the lines of which are
               prefixed by the value of the variable indentprefix, taking into account quote-chars and
               quote-fold.  If set to the empty value, the quoted message will be preceded and followed by
               the expansions of the values of quote-inject-head and quote-inject-tail, respectively.  None
               of the headers of the quoted message is included in the quote if the value equals
               ‘noheading’, and only the headers selected by the ‘type’ headerpick selection are put above
               the message body for ‘headers’, whereas all headers and all MIME parts are included for
               ‘allheaders’.  Also see quote-as-attachment and ~Q, one of the COMMAND ESCAPES.

     quote-as-attachment
               (Boolean) Add the original message in its entirety as a ‘message/rfc822’ MIME attachment
               when replying to a message.  Note this works regardless of the setting of quote.

     quote-chars
               Can be set to a string consisting of non-whitespace ASCII characters which shall be treated
               as quotation leaders, the default being ‘>|}:’.

     quote-fold
               [Option] Can be set in addition to indentprefix, and creates a more fancy quotation in that
               leading quotation characters (quote-chars) are compressed and overlong lines are folded.
               quote-fold can be set to either one, two or three (space separated) numeric values, which
               are interpreted as the maximum (goal) and the minimum line length, respectively, in a spirit
               rather equal to the fmt(1) program, but line- instead of paragraph-based.  The third value
               is used as the maximum line length instead of the first if no better break point can be
               found; it is ignored unless it is larger than the minimum and smaller than the maximum.  If
               not set explicitly the minimum will reflect the goal algorithmically.  The goal cannot be
               smaller than the length of indentprefix plus some additional pad; necessary adjustments take
               place silently.

     quote-inject-head, quote-inject-tail
               The strings to put before and after the text of a quoted message, respectively.  The former
               defaults to ‘%f wrote:\n\n’.  Special format directives will be expanded if possible, and if
               so configured the output will be folded according to quote-fold.  Format specifiers in the
               given strings start with a percent sign ‘%’ and expand values of the original message, un‐
               less noted otherwise.  Note that names and addresses are not subject to the setting of
               showto.  Valid format specifiers are:

               ‘%%’      A plain percent sign.
               ‘%a’      The address(es) of the sender(s).
               ‘%d’      The date found in the ‘Date:’ header of the message when datefield is set (the de‐
                         fault), otherwise the date when the message was received.  Formatting can be con‐
                         trolled by assigning a strftime(3) format string to datefield (and
                         datefield-markout-older).
               ‘%f’      The full name(s) (name and address, as given) of the sender(s).
               ‘%i’      The ‘Message-ID:’.
               ‘%n’      The real name(s) of the sender(s) if there is one and showname allows usage, the
                         address(es) otherwise.
               ‘%r’      The senders real name(s) if there is one, the address(es) otherwise.

     r-option-implicit
               (Boolean) Setting this option evaluates the contents of from (or, if that contains multiple
               addresses, sender) and passes the results onto the used (file-based) MTA as described for
               the -r option (empty argument case).

     recipients-in-cc
               (Boolean) When doing a reply, the original ‘From:’ and ‘To:’ are by default merged into the
               new ‘To:’.  If this variable is set, only the original ‘From:’ ends in the new ‘To:’, the
               rest is merged into ‘Cc:’.

     record    Unless this variable is defined, no copies of outgoing mail will be saved.  If defined it
               gives the pathname, subject to the usual Filename transformations, of a folder where all
               new, replied-to or forwarded messages are saved: when saving to this folder fails the mes‐
               sage is not sent, but instead saved to DEAD.  The standard defines that relative (fully ex‐
               panded) paths are to be interpreted relative to the current directory (cwd), to force inter‐
               pretation relative to folder outfolder needs to be set in addition.

     record-files
               (Boolean) If this variable is set the meaning of record will be extended to cover messages
               which target only file and pipe recipients (see expandaddr).  These address types will not
               appear in recipient lists unless add-file-recipients is also set.

     record-resent
               (Boolean) If this variable is set the meaning of record will be extended to also cover the
               resend and Resend commands.

     reply-in-same-charset
               (Boolean) If this variable is set S-nail first tries to use the same character set of the
               original message for replies.  If this fails, the mechanism described in Character sets is
               evaluated as usual.

     reply-strings
               Can be set to a comma-separated list of (case-insensitive according to ASCII rules) strings
               which shall be recognized in addition to the built-in strings as ‘Subject:’ reply message
               indicators – built-in are ‘Re:’, which is mandated by RFC 5322, as well as the german ‘Aw:’,
               ‘Antw:’, and the ‘Wg:’ which often has been seen in the wild; I.e., the separating colon has
               to be specified explicitly.

     reply-to  A list of addresses to put into the ‘Reply-To:’ field of the message header.  Members of
               this list are handled as if they were in the alternates list.

     replyto   [Obsolete] Variant of reply-to.

     reply-to-honour
               Controls whether a ‘Reply-To:’ header is honoured when replying to a message via reply or
               Lreply.  This is a quadoption; if set without a value it defaults to “yes”.

     rfc822-body-from_
               (Boolean) This variable can be used to force displaying a so-called ‘From_’ line for mes‐
               sages that are embedded into an envelope mail via the ‘message/rfc822’ MIME mechanism, for
               more visual convenience.

     save      (Boolean) Enable saving of (partial) messages in DEAD upon interrupt or delivery error.

     screen    The number of lines that represents a “screenful” of lines, used in headers summary display,
               from searching, message topline display and scrolling via z.  If this variable is not set
               S-nail falls back to a calculation based upon the detected terminal window size and the baud
               rate: the faster the terminal, the more will be shown.  Overall screen dimensions and pager
               usage is influenced by the environment variables COLUMNS and LINES and the variable crt.

     searchheaders
               (Boolean) Expand message-list specifiers in the form ‘/x:y’ to all messages containing the
               substring “y” in the header field ‘x’.  The string search is case insensitive.

     sendcharsets
               [Option] A comma-separated list of character set names that can be used in outgoing internet
               mail.  The value of the variable charset-8bit is automatically appended to this list of
               character sets.  If no character set conversion capabilities are compiled into S-nail then
               the only supported charset is ttycharset.  Also see sendcharsets-else-ttycharset and refer
               to the section Character sets for the complete picture of character set conversion in
               S-nail.

     sendcharsets-else-ttycharset
               (Boolean)[Option] If this variable is set, but sendcharsets is not, then S-nail acts as if
               sendcharsets had been set to the value of the variable ttycharset.  In effect this combina‐
               tion passes through the message data in the character set of the current locale encoding:
               therefore mail message text will be (assumed to be) in ISO-8859-1 encoding when send from
               within a ISO-8859-1 locale, and in UTF-8 encoding when send from within an UTF-8 locale.

               The 8-bit fallback charset-8bit never comes into play as ttycharset is implicitly assumed to
               be 8-bit and capable to represent all files the user may specify (as is the case when no
               character set conversion support is available in S-nail and the only supported character set
               is ttycharset: Character sets).  This might be a problem for scripts which use the suggested
               ‘LC_ALL=C’ setting, since in this case the character set is US-ASCII by definition, so that
               it is better to also override ttycharset, then.

     sender    An address that is put into the ‘Sender:’ field of outgoing messages, quoting RFC 5322: the
               mailbox of the agent responsible for the actual transmission of the message.  This field
               should normally not be used unless the from field contains more than one address, on which
               case it is required.  Dependent on the context this address is handled as if it were in the
               list of alternates.  Also see -r, r-option-implicit.

     sendmail  [Obsolete] Predecessor of mta.

     sendmail-arguments
               [Obsolete] Predecessor of mta-arguments.

     sendmail-no-default-arguments
               [Obsolete](Boolean) Predecessor of mta-no-default-arguments.

     sendmail-progname
               [Obsolete] Predecessor of mta-argv0.

     sendwait  (Boolean) When sending a message wait until the mta (including the built-in SMTP one) exits
               before accepting further commands.  Only with this variable set errors reported by the MTA
               will be recognizable!  If the MTA returns a non-zero exit status, the exit status of S-nail
               will also be non-zero.

     showlast  (Boolean) This setting causes S-nail to start at the last message instead of the first one
               when opening a mail folder, as well as with from and headers.

     showname  (Boolean) Causes S-nail to use the sender's real name instead of the plain address in the
               header field summary and in message specifications.

     showto    (Boolean) Causes the recipient of the message to be shown in the header summary if the mes‐
               sage was sent by the user.

     Sign      The value backing ~A, one of the COMMAND ESCAPES.  Also see message-inject-tail,
               on-compose-leave and on-compose-splice.

     sign      The value backing ~a, one of the COMMAND ESCAPES.  Also see message-inject-tail,
               on-compose-leave and on-compose-splice.

     signature
               [Obsolete] Please use on-compose-splice or on-compose-splice-shell or on-compose-leave and
               (if necessary) message-inject-tail instead!

     skipemptybody
               (Boolean) If an outgoing message does not contain any text in its first or only message
               part, do not send it but discard it silently (see also the command line option -E).

     smime-ca-dir, smime-ca-file
               [Option] Specify the location of trusted CA certificates in PEM (Privacy Enhanced Mail) for
               the purpose of verification of S/MIME signed messages.  tls-ca-dir documents the necessary
               preparation steps to use the former.  The set of CA certificates which are built into the
               TLS library can be explicitly turned off by setting smime-ca-no-defaults, and further fine-
               tuning is possible via smime-ca-flags.

     smime-ca-flags
               [Option] Can be used to fine-tune behaviour of the X509 CA certificate storage, and the cer‐
               tificate verification that is used.  The actual values and their meanings are documented for
               tls-ca-flags.

     smime-ca-no-defaults
               (Boolean)[Option] Do not load the default CA locations that are built into the used to TLS
               library to verify S/MIME signed messages.

     smime-cipher-USER@HOST, smime-cipher
               [Option] Specifies the cipher to use when generating S/MIME encrypted messages (for the
               specified account).  RFC 5751 mandates a default of ‘aes128’ (AES-128 CBC).  Possible values
               are (case-insensitive and) in decreasing cipher strength: ‘aes256’ (AES-256 CBC), ‘aes192’
               (AES-192 CBC), ‘aes128’ (AES-128 CBC), ‘des3’ (DES EDE3 CBC, 168 bits; default if ‘aes128’
               is not available) and ‘des’ (DES CBC, 56 bits).

               The actually available cipher algorithms depend on the cryptographic library that S-nail
               uses.  [Option] Support for more cipher algorithms may be available through dynamic loading
               via, e.g., EVP_get_cipherbyname(3) (OpenSSL) if S-nail has been compiled to support this.

     smime-crl-dir
               [Option] Specifies a directory that contains files with CRLs in PEM format to use when veri‐
               fying S/MIME messages.

     smime-crl-file
               [Option] Specifies a file that contains a CRL in PEM format to use when verifying S/MIME
               messages.

     smime-encrypt-USER@HOST
               [Option] If this variable is set, messages send to the given receiver are encrypted before
               sending.  The value of the variable must be set to the name of a file that contains a cer‐
               tificate in PEM format.

               If a message is sent to multiple recipients, each of them for whom a corresponding variable
               is set will receive an individually encrypted message; other recipients will continue to re‐
               ceive the message in plain text unless the smime-force-encryption variable is set.  It is
               recommended to sign encrypted messages, i.e., to also set the smime-sign variable.

     smime-force-encryption
               (Boolean)[Option] Causes S-nail to refuse sending unencrypted messages.

     smime-sign
               (Boolean)[Option] S/MIME sign outgoing messages with the user's private key and include the
               user's certificate as a MIME attachment.  Signing a message enables a recipient to verify
               that the sender used a valid certificate, that the email addresses in the certificate match
               those in the message header and that the message content has not been altered.  It does not
               change the message text, and people will be able to read the message as usual.  Also see
               smime-sign-cert, smime-sign-include-certs and smime-sign-digest.

     smime-sign-cert-USER@HOST, smime-sign-cert
               [Option] Points to a file in PEM format.  For the purpose of signing and decryption this
               file needs to contain the user's private key, followed by his certificate.

               For message signing ‘USER@HOST’ is always derived from the value of from (or, if that con‐
               tains multiple addresses, sender).  For the purpose of encryption the recipient's public en‐
               cryption key (certificate) is expected; the command certsave can be used to save certifi‐
               cates of signed messages (the section Signed and encrypted messages with S/MIME gives some
               details).  This mode of operation is usually driven by the specialized form.

               When decrypting messages the account is derived from the recipient fields (‘To:’ and ‘Cc:’)
               of the message, which are searched for addresses for which such a variable is set.  S-nail
               always uses the first address that matches, so if the same message is sent to more than one
               of the user's addresses using different encryption keys, decryption might fail.

               For signing and decryption purposes it is possible to use encrypted keys, and the pseudo-
               host(s) ‘USER@HOST.smime-cert-key’ for the private key (and ‘USER@HOST.smime-cert-cert’ for
               the certificate stored in the same file) will be used for performing any necessary password
               lookup, therefore the lookup can be automated via the mechanisms described in On URL syntax
               and credential lookup.  For example, the hypothetical address ‘bob@exam.ple’ could be driven
               with a private key / certificate pair path defined in smime-sign-cert-bob@exam.ple, and
               needed passwords would then be looked up via the pseudo hosts ‘bob@exam.ple.smime-cert-key’
               (and ‘bob@exam.ple.smime-cert-cert’).  To include intermediate certificates, use
               smime-sign-include-certs.

     smime-sign-digest-USER@HOST, smime-sign-digest
               [Option] Specifies the message digestto use when signing S/MIME messages.  Please remember
               that for this use case ‘USER@HOST’ refers to the variable from (or, if that contains multi‐
               ple addresses, sender).  The available algorithms depend on the used cryptographic library,
               but at least one usable builtin algorithm is ensured as a default.  If possible the standard
               RFC 5751 will be violated by using ‘SHA512’ instead of the mandated ‘SHA1’ due to security
               concerns.

               S-nail will try to add built-in support for the following message digests, names are case-
               insensitive: ‘BLAKE2b512’, ‘BLAKE2s256’, ‘SHA3-512’, ‘SHA3-384’, ‘SHA3-256’, ‘SHA3-224’, as
               well as the widely available ‘SHA512’, ‘SHA384’, ‘SHA256’, ‘SHA224’, and the proposed inse‐
               cure ‘SHA1’ and ‘MD5’.  More digests may [Option]ally be available through dynamic loading
               via, e.g., the OpenSSL function EVP_get_digestbyname(3).

     smime-sign-include-certs-USER@HOST, smime-sign-include-certs
               [Option] If used, this is supposed to a consist of a comma-separated list of files, each of
               which containing a single certificate in PEM format to be included in the S/MIME message in
               addition to the smime-sign-cert certificate.  This can be used to include intermediate cer‐
               tificates of the certificate authority, in order to allow the receiver's S/MIME implementa‐
               tion to perform a verification of the entire certificate chain, starting from a local root
               certificate, over the intermediate certificates, down to the smime-sign-cert.  Even though
               top level certificates may also be included in the chain, they won't be used for the verifi‐
               cation on the receiver's side.

               For the purpose of the mechanisms involved here, ‘USER@HOST’ refers to the content of the
               internal variable from (or, if that contains multiple addresses, sender).  The pseudo-host
               ‘USER@HOST.smime-include-certs’ will be used for performing password lookups for these cer‐
               tificates, shall they have been given one, therefore the lookup can be automated via the
               mechanisms described in On URL syntax and credential lookup.

     smime-sign-message-digest-USER@HOST, smime-sign-message-digest
               [Obsolete][Option] Predecessor(s) of smime-sign-digest.

     smtp      [Obsolete][Option] To use the built-in SMTP transport, specify a SMTP URL in mta.  [v15 be‐
               haviour may differ] For compatibility reasons a set smtp is used in preference of mta.

     smtp-auth-USER@HOST, smtp-auth-HOST, smtp-auth
               [Option] Variable chain that controls the SMTP mta authentication method, possible values
               are ‘none’ ([no v15-compat] default), ‘plain’ ([v15-compat] default), ‘login’ as well as the
               [Option]al methods ‘cram-md5’ and ‘gssapi’.  The ‘none’ method does not need any user cre‐
               dentials, ‘gssapi’ requires a user name and all other methods require a user name and a
               password.  See [v15-compat] mta, user and password ([no v15-compat] smtp-auth-password and
               smtp-auth-user).  Note that smtp-auth-HOST is [v15-compat].  [no v15-compat]: Note for
               smtp-auth-USER@HOST: may override dependent on sender address in the variable from.

     smtp-auth-password
               [Option][no v15-compat] Sets the global fallback password for SMTP authentication.  If the
               authentication method requires a password, but neither smtp-auth-password nor a matching
               smtp-auth-password-USER@HOST can be found, S-nail will ask for a password on the user's ter‐
               minal.

     smtp-auth-password-USER@HOST
               [no v15-compat] Overrides smtp-auth-password for specific values of sender addresses, depen‐
               dent upon the variable from.

     smtp-auth-user
               [Option][no v15-compat] Sets the global fallback user name for SMTP authentication.  If the
               authentication method requires a user name, but neither smtp-auth-user nor a matching
               smtp-auth-user-USER@HOST can be found, S-nail will ask for a user name on the user's termi‐
               nal.

     smtp-auth-user-USER@HOST
               [no v15-compat] Overrides smtp-auth-user for specific values of sender addresses, dependent
               upon the variable from.

     smtp-hostname
               [Option][v15-compat] Normally S-nail uses the variable from to derive the necessary
               ‘USER@HOST’ information in order to issue a ‘MAIL FROM:<>’ SMTP mta command.  Setting
               smtp-hostname can be used to use the ‘USER’ from the SMTP account (mta or the user variable
               chain) and the ‘HOST’ from the content of this variable (or, if that is the empty string,
               hostname or the local hostname as a last resort).  This often allows using an address that
               is itself valid but hosted by a provider other than which (in from) is about to send the
               message.  Setting this variable also influences generated ‘Message-ID:’ and ‘Content-ID:’
               header fields.  If the [Option]al IDNA support is available (see idna-disable) variable as‐
               signment is aborted when a necessary conversion fails.

     smtp-use-starttls-USER@HOST, smtp-use-starttls-HOST, smtp-use-starttls
               (Boolean)[Option] Causes S-nail to issue a ‘STARTTLS’ command to make an SMTP mta session
               TLS encrypted, i.e., to enable transport layer security.

     socks-proxy-USER@HOST, socks-proxy-HOST, socks-proxy
               [Option] If this is set to the hostname (SOCKS URL) of a SOCKS5 server then S-nail will
               proxy all of its network activities through it.  This can be used to proxy SMTP, POP3 etc.
               network traffic through the Tor anonymizer, for example.  The following would create a local
               SOCKS proxy on port 10000 that forwards to the machine ‘HOST’, and from which the network
               traffic is actually instantiated:

                     # Create local proxy server in terminal 1 forwarding to HOST
                     $ ssh -D 10000 USER@HOST
                     # Then, start a client that uses it in terminal 2
                     $ s-nail -Ssocks-proxy-USER@HOST=localhost:10000

     spam-interface
               [Option] In order to use any of the spam-related commands (like, e.g., spamrate) the desired
               spam interface must be defined by setting this variable.  Please refer to the manual section
               Handling spam for the complete picture of spam handling in S-nail.  All or none of the fol‐
               lowing interfaces may be available:

               ‘spamc’   Interaction with spamc(1) from the spamassassin(1) (SpamAssassin:
                         http://spamassassin.apache.org) suite.  Different to the generic filter interface
                         S-nail will automatically add the correct arguments for a given command and has
                         the necessary knowledge to parse the program's output.  A default value for
                         spamc-command will have been compiled into the S-nail binary if spamc(1) has been
                         found in PATH during compilation.  Shall it be necessary to define a specific con‐
                         nection type (rather than using a configuration file for that), the variable
                         spamc-arguments can be used as in, e.g., ‘-d server.example.com -p 783’.  It is
                         also possible to specify a per-user configuration via spamc-user.  Note that this
                         interface does not inspect the ‘is-spam’ flag of a message for the command
                         spamforget.

               ‘filter’  generic spam filter support via freely configurable hooks.  This interface is
                         meant for programs like bogofilter(1) and requires according behaviour in respect
                         to the hooks' exit status for at least the command spamrate (‘0’ meaning a message
                         is spam, ‘1’ for non-spam, ‘2’ for unsure and any other return value indicating a
                         hard error); since the hooks can include shell code snippets diverting behaviour
                         can be intercepted as necessary.  The hooks are spamfilter-ham, spamfilter-noham,
                         spamfilter-nospam, spamfilter-rate and spamfilter-spam; the manual section
                         Handling spam contains examples for some programs.  The process environment of the
                         hooks will have the variable MAILX_FILENAME_GENERATED set.  Note that spam score
                         support for spamrate is not supported unless the [Option]tional regular expression
                         support is available and the spamfilter-rate-scanscore variable is set.

     spam-maxsize
               [Option] Messages that exceed this size will not be passed through to the configured
               spam-interface.  If unset or 0, the default of 420000 bytes is used.

     spamc-command
               [Option] The path to the spamc(1) program for the ‘spamc’ spam-interface.  Note that the
               path is not expanded, but used “as is”.  A fallback path will have been compiled into the
               S-nail binary if the executable had been found during compilation.

     spamc-arguments
               [Option] Even though S-nail deals with most arguments for the ‘spamc’ spam-interface auto‐
               matically, it may at least sometimes be desirable to specify connection-related ones via
               this variable, e.g., ‘-d server.example.com -p 783’.

     spamc-user
               [Option] Specify a username for per-user configuration files for the ‘spamc’ spam-interface.
               If this is set to the empty string then S-nail will use the name of the current user.

     spamfilter-ham, spamfilter-noham, spamfilter-nospam, spamfilter-rate, spamfilter-spam
               [Option] Command and argument hooks for the ‘filter’ spam-interface.  The manual section
               Handling spam contains examples for some programs.

     spamfilter-rate-scanscore
               [Option] Because of the generic nature of the ‘filter’ spam-interface spam scores are not
               supported for it by default, but if the [Option]nal regular expression support is available
               then setting this variable can be used to overcome this restriction.  It is interpreted as
               follows: first a number (digits) is parsed that must be followed by a semicolon ‘;’ and an
               extended regular expression.  Then the latter is used to parse the first output line of the
               spamfilter-rate hook, and, in case the evaluation is successful, the group that has been
               specified via the number is interpreted as a floating point scan score.

     ssl-ca-dir-USER@HOST, ssl-ca-dir-HOST, ssl-ca-dir, ssl-ca-file-USER@HOST, ssl-ca-file-HOST,
               ssl-ca-file
               [Obsolete][Option] Predecessors of tls-ca-file, tls-ca-dir.

     ssl-ca-flags-USER@HOST, ssl-ca-flags-HOST, ssl-ca-flags
               [Obsolete][Option] Predecessor of tls-ca-flags.

     ssl-ca-no-defaults-USER@HOST, ssl-ca-no-defaults-HOST, ssl-ca-no-defaults
               [Obsolete](Boolean)[Option] Predecessor of tls-ca-no-defaults.

     ssl-cert-USER@HOST, ssl-cert-HOST, ssl-cert
               [Obsolete][Option] Please use the Certificate slot of tls-config-pairs.

     ssl-cipher-list-USER@HOST, ssl-cipher-list-HOST, ssl-cipher-list
               [Obsolete][Option] Please use the CipherString slot of tls-config-pairs.

     ssl-config-file
               [Obsolete][Option] Predecessor of tls-config-file.

     ssl-config-module-USER@HOST, ssl-config-module-HOST, ssl-config-module
               [Obsolete][Option] Predecessor of tls-config-module.

     ssl-config-pairs-USER@HOST, ssl-config-pairs-HOST, ssl-config-pairs
               [Obsolete][Option] Predecessor of tls-config-pairs.

     ssl-crl-dir, ssl-crl-file
               [Obsolete][Option] Predecessors of tls-crl-dir, tls-crl-file.

     ssl-curves-USER@HOST, ssl-curves-HOST, ssl-curves
               [Obsolete][Option] Please use the Curves slot of tls-config-pairs.

     ssl-features
               [Obsolete][Option](Read-only) Predecessor of tls-features.

     ssl-key-USER@HOST, ssl-key-HOST, ssl-key
               [Obsolete][Option] Please use the PrivateKey slot of tls-config-pairs.

     ssl-method-USER@HOST, ssl-method-HOST, ssl-method
               [Obsolete][Option] Please use the Protocol slot of tls-config-pairs.

     ssl-protocol-USER@HOST, ssl-protocol-HOST, ssl-protocol
               [Obsolete][Option] Please use the Protocol slot of tls-config-pairs.

     ssl-rand-file
               [Obsolete][Option] Predecessor of tls-rand-file.

     ssl-verify-USER@HOST, ssl-verify-HOST, ssl-verify
               [Obsolete][Option] Predecessor of tls-verify.

     stealthmua
               If only set without an assigned value, then this setting inhibits the generation of the
               ‘Message-ID:’, ‘Content-ID:’ and ‘User-Agent:’ header fields that include obvious references
               to S-nail.  There are two pitfalls associated with this: First, the message id of outgoing
               messages is not known anymore.  Second, an expert may still use the remaining information in
               the header to track down the originating mail user agent.  If set to the value ‘noagent’,
               then the mentioned ‘Message-ID:’ and ‘Content-ID:’ suppression does not occur.

     system-mailrc
               (Read-only) The compiled in path of the system wide initialization file one of the Resource
               files: /etc/s-nail.rc.

     termcap   ([Option]) This specifies a comma-separated list of Terminal Information Library
               (libterminfo, -lterminfo) and/or Termcap Access Library (libtermcap, -ltermcap) capabilities
               (see On terminal control and line editor, escape commas with reverse solidus) to be used to
               overwrite or define entries.  Note this variable will only be queried once at program
               startup and can thus only be specified in resource files or on the command line.

               String capabilities form ‘cap=value’ pairs and are expected unless noted otherwise.  Numer‐
               ics have to be notated as ‘cap#number’ where the number is expected in normal decimal nota‐
               tion.  Finally, booleans do not have any value but indicate a true or false state simply by
               being defined or not; this indeed means that S-nail does not support undefining an existing
               boolean.  String capability values will undergo some expansions before use: for one nota‐
               tions like ‘^LETTER’ stand for ‘control-LETTER’, and for clarification purposes ‘\E’ can be
               used to specify ‘escape’ (the control notation ‘^[’ could lead to misreadings when a left
               bracket follows, which it does for the standard CSI sequence); finally three letter octal
               sequences, as in ‘\061’, are supported.  To specify that a terminal supports 256-colours,
               and to define sequences that home the cursor and produce an audible bell, one might write:

                     ? set termcap='Co#256,home=\E[H,bel=^G'

               The following terminal capabilities are or may be meaningful for the operation of the built-
               in line editor or S-nail in general:

               colors or Co
                         max_colors: numeric capability specifying the maximum number of colours.  Note
                         that S-nail does not actually care about the terminal beside that, but always
                         emits ANSI / ISO 6429 escape sequences.
               rmcup or te / smcup or ti
                         exit_ca_mode and enter_ca_mode, respectively: exit and enter the alternative
                         screen ca-mode, effectively turning S-nail into a fullscreen application.  This
                         must be enabled explicitly by setting termcap-ca-mode.
               smkx or ks / rmkx or ke
                         keypad_xmit and keypad_local, respectively: enable and disable the keypad.  This
                         is always enabled if available, because it seems even keyboards without keypads
                         generate other key codes for, e.g., cursor keys in that case, and only if enabled
                         we see the codes that we are interested in.
               ed or cd  clr_eos: clear the screen.
               clear or cl
                         clear_screen: clear the screen and home cursor.  (Will be simulated via ho plus
                         cd.)
               home or ho
                         cursor_home: home cursor.
               el or ce  clr_eol: clear to the end of line.  (Will be simulated via ch plus repetitions of
                         space characters.)
               hpa or ch
                         column_address: move the cursor (to the given column parameter) in the current
                         row.  (Will be simulated via cr plus nd.)
               cr        carriage_return: move to the first column in the current row.  The default built-
                         in fallback is ‘\r’.
               cub1 or le
                         cursor_left: move the cursor left one space (non-destructively).  The default
                         built-in fallback is ‘\b’.
               cuf1 or nd
                         cursor_right: move the cursor right one space (non-destructively).  The default
                         built-in fallback is ‘\E[C’, which is used by most terminals.  Less often occur
                         ‘\EC’ and ‘\EOC’.

               Many more capabilities which describe key-sequences are documented for bind.

     termcap-ca-mode
               [Option] Allow usage of the exit_ca_mode and enter_ca_mode terminal capabilities, see
               termcap.  Note this variable will only be queried once at program startup and can thus only
               be specified in resource files or on the command line.

     termcap-disable
               [Option] Disable any interaction with a terminal control library.  If set only some generic
               fallback built-ins and possibly the content of termcap describe the terminal to S-nail.
               Note this variable will only be queried once at program startup and can thus only be speci‐
               fied in resource files or on the command line.

     tls-ca-dir-USER@HOST, tls-ca-dir-HOST, tls-ca-dir, tls-ca-file-USER@HOST, tls-ca-file-HOST,
               tls-ca-file
               [Option] Directory and file, respectively, for pools of trusted CA certificates in PEM (Pri‐
               vacy Enhanced Mail) format, for the purpose of verification of TLS server certificates.
               Concurrent use is possible, the file is loaded once needed first, the directory lookup is
               performed anew as a last resort whenever necessary.  The CA certificate pool built into the
               TLS library can be disabled via tls-ca-no-defaults, further fine-tuning is possible via
               tls-ca-flags.  Note the directory search variant requires the certificate files to adhere
               special filename conventions, please see SSL_CTX_load_verify_locations(3) and verify(1) (or
               c_rehash(1)).

     tls-ca-flags-USER@HOST, tls-ca-flags-HOST, tls-ca-flags
               [Option] Can be used to fine-tune behaviour of the X509 CA certificate storage, and the cer‐
               tificate verification that is used (also see tls-verify).  The value is expected to consist
               of a comma-separated list of configuration directives, with any intervening whitespace being
               ignored.  The directives directly map to flags that can be passed to
               X509_STORE_set_flags(3), which are usually defined in a file openssl/x509_vfy.h, and the
               availability of which depends on the used TLS library version: a directive without mapping
               is ignored (error log subject to debug).  Directives currently understood (case-insensi‐
               tively) include:

               no-alt-chains
                         If the initial chain is not trusted, do not attempt to build an alternative chain.
                         Setting this flag will make OpenSSL certificate verification match that of older
                         OpenSSL versions, before automatic building and checking of alternative chains has
                         been implemented; also see trusted-first.
               no-check-time
                         Do not check certificate/CRL validity against current time.
               partial-chain
                         By default partial, incomplete chains which cannot be verified up to the chain
                         top, a self-signed root certificate, will not verify.  With this flag set, a chain
                         succeeds to verify if at least one signing certificate of the chain is in any of
                         the configured trusted stores of CA certificates.  The OpenSSL manual page
                         SSL_CTX_load_verify_locations(3) gives some advise how to manage your own trusted
                         store of CA certificates.
               strict    Disable workarounds for broken certificates.
               trusted-first
                         Try building a chain using issuers in the trusted store first to avoid problems
                         with server-sent legacy intermediate certificates.  Newer versions of OpenSSL sup‐
                         port alternative chain checking and enable it by default, resulting in the same
                         behaviour; also see no-alt-chains.

     tls-ca-no-defaults-USER@HOST, tls-ca-no-defaults-HOST, tls-ca-no-defaults
               (Boolean)[Option] Do not load the default CA locations that are built into the used to TLS
               library to verify TLS server certificates.

     tls-config-file
               [Option] If this variable is set CONF_modules_load_file(3) (if announced via
               ‘+modules-load-file’ in tls-features) is used to allow resource file based configuration of
               the TLS library.  This happens once the library is used first, which may also be early dur‐
               ing startup (logged with verbose)!  If a non-empty value is given then the given file, after
               performing Filename transformations, will be used instead of the TLS libraries global de‐
               fault, and it is an error if the file cannot be loaded.  The application name will always be
               passed as ‘s-nail’.  Some TLS libraries support application-specific configuration via re‐
               source files loaded like this, please see tls-config-module.

     tls-config-module-USER@HOST, tls-config-module-HOST, tls-config-module
               [Option] If file based application-specific configuration via tls-config-file is available,
               announced as ‘+ctx-config’ by tls-features, indicating availability of SSL_CTX_config(3),
               then, it becomes possible to use a central TLS configuration file for all programs, includ‐
               ing s-nail, e.g.:

                     # Register a configuration section for s-nail
                     s-nail = mailx_master
                     # The top configuration section creates a relation
                     # in between dynamic SSL configuration and an actual
                     # program specific configuration section
                     [mailx_master]
                     ssl_conf = mailx_tls_config
                     # Well that actual program specific configuration section
                     # now can map individual tls-config-module names to sections,
                     # e.g., tls-config-module=account_xy
                     [mailx_tls_config]
                     account_xy = mailx_account_xy
                     account_yz = mailx_account_yz
                     [mailx_account_xy]
                     MinProtocol = TLSv1.2
                     Curves=P-521
                     [mailx_account_yz]
                     CipherString = TLSv1.2:!aNULL:!eNULL:
                     MinProtocol = TLSv1.1
                     Options = Bugs

     tls-config-pairs-USER@HOST, tls-config-pairs-HOST, tls-config-pairs
               [Option] The value of this variable chain will be interpreted as a comma-separated list of
               directive/value pairs.  Directives and values need to be separated by equals signs ‘=’, any
               whitespace surrounding pair members is removed.  Keys are (usually) case-insensitive.  Dif‐
               ferent to when placing these pairs in a tls-config-module section of a tls-config-file, com‐
               mas ‘,’ need to be escaped with a reverse solidus ‘\’ when included in pairs; also differ‐
               ent: if the equals sign ‘=’ is preceded with an asterisk ‘*’ Filename transformations will
               be performed on the value; it is an error if these fail.  Unless proper support is announced
               by tls-features (‘+conf-ctx’) only the keys below are supported, otherwise the pairs will be
               used directly as arguments to the function SSL_CONF_cmd(3).

               Certificate   Filename of a TLS client certificate (chain) required by some servers.  Fall‐
                             back support via SSL_CTX_use_certificate_chain_file(3).  Filename
                             transformations are performed.  Note: if you use this you need to specify the
                             private key via PrivateKey, tls-key will not be used!
               CipherString  A list of ciphers for TLS connections, see ciphers(1).  By default no list of
                             ciphers is set, resulting in a Protocol-specific list of ciphers (the protocol
                             standards define lists of acceptable ciphers; possibly cramped by the used TLS
                             library).  Fallback support via SSL_CTX_set_cipher_list(3).
               Ciphersuites  A list of ciphers used for TLSv1.3 connections, see ciphers(1).  These will be
                             joined onto the list of ciphers from CipherString.  Available if tls-features
                             announces ‘+ctx-set-ciphersuites’, as necessary via
                             SSL_CTX_set_ciphersuites(3).
               Curves        A list of supported elliptic curves, if applicable.  By default no curves are
                             set.  Fallback support via SSL_CTX_set1_curves_list(3), if available.
               MaxProtocol, MinProtocol
                             The maximum and minimum supported TLS versions, respectively.  Available if
                             tls-features announces ‘+ctx-set-maxmin-proto’, as necessary via
                             SSL_CTX_set_max_proto_version(3) and SSL_CTX_set_min_proto_version(3); these
                             fallbacks use an internal parser which understands the strings ‘SSLv3’,
                             ‘TLSv1’, ‘TLSv1.1’, ‘TLSv1.2’, ‘TLSv1.3’, and the special value ‘None’, which
                             disables the given limit.
               Options       Various flags to set.  Fallback via SSL_CTX_set_options(3), in which case any
                             other value but (exactly) ‘Bugs’ results in an error.
               PrivateKey    Filename of the private key in PEM format of a TLS client certificate.  If un‐
                             set, the name of the certificate file is used.  Filename transformations are
                             performed.  Fallback via SSL_CTX_use_PrivateKey_file(3).  Note: if you use
                             this you need to specify the certificate (chain) via Certificate, tls-cert
                             will not be used!
               Protocol      The used TLS protocol.  If tls-features announces ‘+conf-ctx’ or
                             ‘ctx-set-maxmin-proto’ then using MaxProtocol and MinProtocol is preferable.
                             Fallback is SSL_CTX_set_options(3), driven via an internal parser which under‐
                             stands the strings ‘SSLv3’, ‘TLSv1’, ‘TLSv1.1’, ‘TLSv1.2’, ‘TLSv1.3’, and the
                             special value ‘ALL’.  Multiple protocols may be given as a comma-separated
                             list, any whitespace is ignored, an optional plus sign ‘+’ prefix enables, a
                             hyphen-minus ‘-’ prefix disables a protocol, so that ‘-ALL, TLSv1.2’ enables
                             only the TLSv1.2 protocol.

     tls-crl-dir, tls-crl-file
               [Option] Specify a directory / a file, respectively, that contains a CRL in PEM format to
               use when verifying TLS server certificates.

     tls-features
               [Option](Read-only) This expands to a comma separated list of the TLS library identity and
               optional SSL library features.  Currently supported identities are ‘libressl’ (LibreSSL) ,
               ‘libssl-0x10100’ (OpenSSL v1.1.x series) and ‘libssl-0x10000’ (elder OpenSSL series, other
               clones).  Optional features are preceded with a plus sign ‘+’ when available, and with a hy‐
               phen-minus ‘-’ otherwise.

               Currently known features are ‘modules-load-file’ (tls-config-file), ‘conf-ctx’
               (tls-config-pairs), ‘ctx-config’ (tls-config-module), ‘ctx-set-maxmin-proto’
               (tls-config-pairs), ‘ctx-set-ciphersuites’ (the Ciphersuites slot of tls-config-pairs).

     tls-fingerprint-USER@HOST, tls-fingerprint-HOST, tls-fingerprint
               [Option] It is possible to replace the verification of the connection peer certificate
               against the entire local pool of CAs (for more see Encrypted network communication) with the
               comparison against a precalculated certificate message digest, the so-called fingerprint, to
               be specified as the used tls-fingerprint-digest.  This fingerprint can be calculated with,
               e.g., ‘tls fingerprint HOST’.

     tls-fingerprint-digest-USER@HOST, tls-fingerprint-digest-HOST, tls-fingerprint-digest
               [Option] The message digest to be used when creating TLS certificate fingerprints, the de‐
               faults, if available, in test order, being ‘BLAKE2s256’, ‘SHA256’.  For the complete list of
               digest algorithms refer to smime-sign-digest.

     tls-rand-file
               [Option] Gives the filename to a file with random entropy data, see RAND_load_file(3).  If
               this variable is not set, or set to the empty string, or if the Filename transformations
               fail, then RAND_file_name(3) will be used to create the filename.  If the SSL PRNG was
               seeded successfully The file will be updated (RAND_write_file(3)) if and only if seeding and
               buffer stirring succeeds.

     tls-verify-USER@HOST, tls-verify-HOST, tls-verify
               [Option] Variable chain that sets the action to be performed if an error occurs during TLS
               server certificate validation against the specified or default trust stores tls-ca-dir,
               tls-ca-file, or the TLS library built-in defaults (unless usage disallowed via
               tls-ca-no-defaults), and as fine-tuned via tls-ca-flags.  Valid (case-insensitive) values
               are ‘strict’ (fail and close connection immediately), ‘ask’ (ask whether to continue on
               standard input), ‘warn’ (show a warning and continue), ‘ignore’ (do not perform validation).
               The default is ‘ask’.

     toplines  If defined, gives the number of lines of a message to be displayed with the command top; if
               unset, the first five lines are printed, if set to 0 the variable screen is inspected.  If
               the value is negative then its absolute value will be used for unsigned right shifting (see
               vexpr) the screen height.

     topsqueeze
               (Boolean) If set then the top command series will strip adjacent empty lines and quotations.

     ttycharset
               The character set of the terminal S-nail operates on, and the one and only supported charac‐
               ter set that S-nail can use if no character set conversion capabilities have been compiled
               into it, in which case it defaults to ISO-8859-1.  Otherwise it defaults to UTF-8.  Suffi‐
               cient locale support provided the default will be preferably deduced from the locale envi‐
               ronment if that is set (e.g., LC_CTYPE, see there for more); runtime locale changes will be
               reflected by ttycharset except during the program startup phase and if -S had been used to
               freeze the given value.  Refer to the section Character sets for the complete picture about
               character sets.

     typescript-mode
               (Boolean) A special multiplex variable that disables all variables and settings which result
               in behaviour that interferes with running S-nail in script(1), e.g., it sets colour-disable,
               line-editor-disable and (before startup completed only) termcap-disable.  Unsetting it does
               not restore the former state of the covered settings.

     umask     For a safe-by-default policy the process file mode creation mask umask(2) will be set to
               ‘0077’ on program startup by default.  Child processes inherit the file mode creation mask
               of their parent, and by setting this variable to an empty value no change will be applied,
               and the inherited value will be used.  Otherwise the given value will be made the new file
               mode creation mask.

     user-HOST, user
               [v15-compat] Variable chain that sets a global fallback user name, which is used in case
               none has been given in the protocol and account-specific URL.  This variable defaults to the
               name of the user who runs S-nail.

     v15-compat
               (Boolean) Setting this enables upward compatibility with S-nail version 15.0 in respect to
               which configuration options are available and how they are handled.  This manual uses
               [v15-compat] and [no v15-compat] to refer to the new and the old way of doing things, re‐
               spectively.

     verbose   (Boolean) This setting, also controllable via the command line option -v, causes S-nail to
               be more verbose, e.g., it will display obsoletion warnings and TLS certificate chains.  Even
               though marked (Boolean) this option may be set twice in order to increase the level of ver‐
               bosity even more, in which case even details of the actual message delivery and protocol
               conversations are shown.  A single noverbose is sufficient to disable verbosity as such.

     version, version-date, version-hexnum, version-major, version-minor, version-update
               (Read-only) S-nail version information: the first variable is a string with the complete
               version identification, the second the release date in ISO 8601 notation without time.  The
               third is a 32-bit hexadecimal number with the upper 8 bits storing the major, followed by
               the minor and update version numbers which occupy 12 bits each.  The latter three variables
               contain only decimal digits: the major, minor and update version numbers.  The output of the
               command version will include this information.

     writebackedited
               If this variable is set messages modified using the edit or visual commands are written back
               to the current folder when it is quit; it is only honoured for writable folders in MBOX for‐
               mat, though.  Note that the editor will be pointed to the raw message content in that case,
               i.e., neither MIME decoding nor decryption will have been performed, and proper RFC 4155
               ‘From_’ quoting of newly added or edited content is also left as an exercise to the user.

ENVIRONMENT
     The term “environment variable” should be considered an indication that these variables are either
     standardized as vivid parts of process environments, or that they are commonly found in there.  The
     process environment is inherited from the sh(1) once S-nail is started, and unless otherwise explic‐
     itly noted handling of the following variables transparently integrates into that of the INTERNAL
     VARIABLES from S-nail's point of view.  This means that, e.g., they can be managed via set and unset,
     causing automatic program environment updates (to be inherited by newly created child processes).

     In order to integrate other environment variables equally they need to be imported (linked) with the
     command environ.  This command can also be used to set and unset non-integrated environment variables
     from scratch, sufficient system support provided.  The following example, applicable to a POSIX shell,
     sets the COLUMNS environment variable for S-nail only, and beforehand exports the EDITOR in order to
     affect any further processing in the running shell:

           $ EDITOR="vim -u ${HOME}/.vimrc"
           $ export EDITOR
           $ COLUMNS=80 s-nail -R

     COLUMNS   The user's preferred width in column positions for the terminal screen or window.  Queried
               and used once on program startup, actively managed for child processes and the MLE (see On
               terminal control and line editor) in interactive mode thereafter.  Ignored in non-interac‐
               tive mode, which always uses 80 columns, unless in -# batch mode.

     DEAD      The name of the (mailbox) file to use for saving aborted messages if save is set; this de‐
               faults to ~/dead.letter.  If the variable debug is set no output will be generated, other‐
               wise the contents of the file will be replaced.

     EDITOR    Pathname of the text editor to use for the edit command and ~e (see COMMAND ESCAPES); VISUAL
               is used for a more display oriented editor.

     HOME      The user's home directory.  This variable is only used when it resides in the process envi‐
               ronment.  The calling user's home directory will be used instead if this directory does not
               exist, is not accessible or cannot be read; it will always be used for the root user.  (No
               test for being writable is performed to allow usage by non-privileged users within read-only
               jails, but dependent on the variable settings this directory is a default write target, e.g.
               for DEAD, MBOX and more.)

     LC_ALL, LC_CTYPE, LANG
               [Option] The (names in lookup order of the) locale(7) (and / or see setlocale(3)) which in‐
               dicates the used Character sets.  Runtime changes trigger automatic updates of the entire
               locale system, which includes updating ttycharset (except during startup if the variable has
               been frozen via -S).

     LINES     The user's preferred number of lines on a page or the vertical screen or window size in
               lines.  Queried and used once on program startup, actively managed for child processes in
               interactive mode thereafter.  Ignored in non-interactive mode, which always uses 24 lines,
               unless in
               batch mode.

     LISTER    Pathname of the directory lister to use in the folders command when operating on local mail‐
               boxes.  Default is ls(1) (path search through SHELL).

     LOGNAME   Upon startup S-nail will actively ensure that this variable refers to the name of the user
               who runs S-nail, in order to be able to pass a verified name to any newly created child
               process.

     MAIL      Is used as the user's primary system mailbox unless inbox is set.  This is assumed to be an
               absolute pathname.  If this environmental fallback is also not set, a built-in compile-time
               default is used.

     MAILCAPS  [Option] Overrides the default path search for The Mailcap files, which is defined in the
               standard RFC 1524 as ‘~/.mailcap:/etc/mailcap:/usr/etc/mailcap:/usr/local/etc/mailcap’.  (‐
               S-nail makes it a configuration option, however.)  Note this is not a search path, but a
               path search.

     MAILRC    Is used as a startup file instead of ~/.mailrc if set.  In order to avoid side-effects from
               configuration files scripts should either set this variable to /dev/null or the -: command
               line option should be used.

     MAILX_NO_SYSTEM_RC
               If this variable is set then reading of /etc/s-nail.rc (a.k.a. system-mailrc) at startup is
               inhibited, i.e., the same effect is achieved as if S-nail had been started up with the op‐
               tion -: (and according argument) or -n.  This variable is only used when it resides in the
               process environment.

     MBOX      The name of the user's secondary mailbox file.  A logical subset of the special Filename
               transformations (also see file) are supported.  The default is ~/mbox.  Traditionally this
               MBOX is used as the file to save messages from the primary system mailbox that have been
               read.  Also see Message states.

     NETRC     [v15-compat][Option] This variable overrides the default location of the user's ~/.netrc
               file.

     PAGER     Pathname of the program to use for backing the command more, and when the crt variable en‐
               forces usage of a pager for output.  The default paginator is more(1) (path search through
               SHELL).

               S-nail inspects the contents of this variable: if its contains the string “less” then a non-
               existing environment variable LESS will be set to ‘Ri’, likewise for “lv” LV will optionally
               be set to ‘-c’.  Alse see colour-pager.

     PATH      A colon-separated list of directories that is searched by the shell when looking for com‐
               mands, e.g., ‘/bin:/usr/bin:/usr/local/bin’.

     POSIXLY_CORRECT
               This variable is automatically looked for upon startup, see posix for more.

     SHELL     The shell to use for the commands !, shell, the ~! COMMAND ESCAPES and when starting subpro‐
               cesses.  A default shell is used if this environment variable is not defined.

     SOURCE_DATE_EPOCH
               Specifies a time in seconds since the Unix epoch (1970-01-01) to be used in place of the
               current time.  This variable is looked up upon program startup, and its existence will
               switch S-nail to a reproducible mode (https://reproducible-builds.org) which uses determin‐
               istic random numbers, a special fixated pseudo LOGNAME and more.  This operation mode is
               used for development and by software packagers.  [v15 behaviour may differ] Currently an in‐
               valid setting is only ignored, rather than causing a program abortion.

                     $ SOURCE_DATE_EPOCH=`date +%s` s-nail

     TERM      [Option] The terminal type for which output is to be prepared.  For extended colour and font
               control please refer to Coloured display, and for terminal management in general to On
               terminal control and line editor.

     TMPDIR    Except for the root user this variable defines the directory for temporary files to be used
               instead of /tmp (or the given compile-time constant) if set, existent, accessible as well as
               read- and writable.  This variable is only used when it resides in the process environment,
               but S-nail will ensure at startup that this environment variable is updated to contain a us‐
               able temporary directory.

     USER      Identical to LOGNAME (see there), but this variable is not standardized, should therefore
               not be used, and is only corrected if already set.

     VISUAL    Pathname of the text editor to use for the visual command and ~v (see COMMAND ESCAPES);
               EDITOR is used for a less display oriented editor.

FILES
     ~/.mailrc
               User-specific file giving initial commands, one of the Resource files.  The actual value is
               read from MAILRC.

     /etc/s-nail.rc
               System wide initialization file, one of the Resource files.  The actual value is read from
               system-mailrc.

     ~/.mailcap
               [Option] Personal MIME type handler definition file, see The Mailcap files.  This location
               is part of the RFC 1524 standard search path, which is a configuration option and can be
               overridden via MAILCAPS.

     /etc/mailcap
               [Option] System wide MIME type handler definition file, see The Mailcap files.  This loca‐
               tion is part of the RFC 1524 standard search path, which is a configuration option and can
               be overridden via

     ~/mbox    The default value for MBOX.

     ~/.mime.types
               Personal MIME types, see The mime.types files.

     /etc/mime.types
               System wide MIME types, see The mime.types files.

     ~/.netrc  [v15-compat][Option] The default location of the user's .netrc file – the section The .netrc
               file documents the file format.  The actually used path can be overridden via NETRC.

     /dev/null
               The data sink null(4).

   Resource files
     Upon startup S-nail reads in several resource files, in order:

     /etc/s-nail.rc
               System wide initialization file (system-mailrc).  Reading of this file can be suppressed,
               either by using the -: (and according argument) or -n command line options, or by setting
               the ENVIRONMENT variable MAILX_NO_SYSTEM_RC.

     ~/.mailrc
               File giving initial commands.  A different file can be chosen by setting the ENVIRONMENT
               variable MAILRC.  Reading of this file can be suppressed with the -: command line option.

     mailx-extra-rc
               Defines a startup file to be read after all other resource files.  It can be used to specify
               settings that are not understood by other mailx(1) implementations, for example.  This vari‐
               able is only honoured when defined in a resource file, e.g., it is one of the INTERNAL
               VARIABLES.

     The content of these files is interpreted as follows:

     •   The whitespace characters space, tabulator and newline, as well as those defined by the variable
         ifs, are removed from the beginning and end of input lines.
     •   Empty lines are ignored.
     •   Any other line is interpreted as a command.  It may be spread over multiple input lines if the
         newline character is “escaped” by placing a reverse solidus character ‘\’ as the last character of
         the line; whereas any leading whitespace of follow lines is ignored, trailing whitespace before a
         escaped newline remains in the input.
     •   If the line (content) starts with the number sign ‘#’ then it is a comment-command and also ig‐
         nored.  (The comment-command is a real command, which does nothing, and therefore the usual follow
         lines mechanism applies!)

     Unless S-nail is about to enter interactive mode syntax errors that occur while loading these files
     are treated as errors and cause program exit.  More files with syntactically equal content can be
     sourceed.  The following, saved in a file, would be an examplary content:

            # This line is a comment command.  And y\
               es, it is really continued here.
           set debug \
               verbose
               set editheaders

   The mime.types files
     As stated in HTML mail and MIME attachments S-nail needs to learn about MIME (Multipurpose Internet
     Mail Extensions) media types in order to classify message and attachment content.  One source for them
     are mime.types files, the loading of which can be controlled by setting the variable
     mimetypes-load-control.  Another is the command mimetype, which also offers access to S-nails MIME
     type cache.  mime.types files have the following syntax:

           type/subtype extension [extension ...]
           # E.g., text/html html htm

     where ‘type/subtype’ define the MIME media type, as standardized in RFC 2046: ‘type’ is used to de‐
     clare the general type of data, while the ‘subtype’ specifies a specific format for that type of data.
     One or multiple filename ‘extension’s, separated by whitespace, can be bound to the media type format.
     Comments may be introduced anywhere on a line with a number sign ‘#’, causing the remaining line to be
     discarded.  S-nail also supports an extended, non-portable syntax in especially crafted files, which
     can be loaded via the alternative value syntax of mimetypes-load-control, and prepends an optional
     ‘type-marker’:

           [type-marker ]type/subtype extension [extension ...]

     The following type markers are supported:

     @         Treat message parts with this content as plain text.
     @t        The same as plain @.
     @h        Treat message parts with this content as HTML tagsoup.  If the [Option]al HTML-tagsoup-to-
               text converter is not available treat the content as plain text instead.
     @H        Likewise @h, but instead of falling back to plain text require an explicit content handler
               to be defined.
     @q        If no handler can be found a text message is displayed which says so.  This can be annoying,
               for example signatures serve a contextual purpose, their content is of no use by itself.
               This marker will avoid displaying the text message.

     Further reading: for sending messages: mimetype, mime-allow-text-controls, mimetypes-load-control.
     For reading etc. messages: HTML mail and MIME attachments, The Mailcap files, mimetype,
     mime-counter-evidence, mimetypes-load-control, pipe-TYPE/SUBTYPE, pipe-EXTENSION.

   The Mailcap files
     This feature is not available in v14.9.0, sorry! RFC 1524 defines a “User Agent Configuration
     Mechanism” which S-nail [Option]ally supports (see HTML mail and MIME attachments).  It defines a file
     format to be used to inform mail user agent programs about the locally-installed facilities for han‐
     dling various data formats, i.e., about commands and how they can be used to display, edit et cetera
     MIME part contents, as well as a default path search that includes multiple possible locations of
     “mailcap” files and the MAILCAPS environment variable that can be used to overwrite that (repeating
     here that it is not a search path, but instead a path search specification).  Any existing files will
     be loaded in sequence, appending any content to the list of MIME type handler directives.

     “Mailcap” files consist of a set of newline separated entries.  Comment lines start with a number sign
     ‘#’ (in the first column!) and are ignored.  Empty lines are also ignored.  All other lines form indi‐
     vidual entries that must adhere to the syntax described below.  To extend a single entry (not comment)
     its line can be continued on follow lines if newline characters are “escaped” by preceding them with
     the reverse solidus character ‘\’.  The standard does not specify how leading whitespace of follow
     lines is to be treated, therefore S-nail retains it.

     “Mailcap” entries consist of a number of semicolon ‘;’ separated fields, and the reverse solidus ‘\’
     character can be used to escape any following character including semicolon and itself.  The first two
     fields are mandatory and must occur in the specified order, the remaining fields are optional and may
     appear in any order.  Leading and trailing whitespace of content is ignored (removed).

     The first field defines the MIME ‘TYPE/SUBTYPE’ the entry is about to handle (case-insensitively, and
     no reverse solidus escaping is possible in this field).  If the subtype is specified as an asterisk
     ‘*’ the entry is meant to match all subtypes of the named type, e.g., ‘audio/*’ would match any audio
     type.  The second field defines the shell command which shall be used to “display” MIME parts of the
     given type; it is implicitly called the view command.

     For data “consuming” shell commands message (MIME part) data is passed via standard input unless the
     given shell command includes one or more instances of the (unquoted) string ‘%s’, in which case these
     instances will be replaced with a temporary filename and the data will have been stored in the file
     that is being pointed to.  Likewise, for data “producing” shell commands data is assumed to be gener‐
     ated on standard output unless the given command includes (one ore multiple) ‘%s’.  In any case any
     given ‘%s’ format is replaced with a(n already) properly quoted filename.  Note that when a command
     makes use of a temporary file via ‘%s’ then S-nail will remove it again, as if the x-mailx-tmpfile,
     x-mailx-tmpfile-fill and x-mailx-tmpfile-unlink flags had been set; see below for more.

     The optional fields either define a shell command or an attribute (flag) value, the latter being a
     single word and the former being a keyword naming the field followed by an equals sign ‘=’ succeeded
     by a shell command, and as usual for any “Mailcap” content any whitespace surrounding the equals sign
     will be removed, too.  Optional fields include the following:

     compose   A program that can be used to compose a new body or body part in the given format.  (Cur‐
               rently unused.)

     composetyped
               Similar to the compose field, but is to be used when the composing program needs to specify
               the ‘Content-type:’ header field to be applied to the composed data.  (Currently unused.)

     edit      A program that can be used to edit a body or body part in the given format.  (Currently un‐
               used.)

     print     A program that can be used to print a message or body part in the given format.  (Currently
               unused.)

     test      Specifies a program to be run to test some condition, e.g., the machine architecture, or the
               window system in use, to determine whether or not this mailcap entry applies.  If the test
               fails, a subsequent mailcap entry should be sought; also see x-mailx-test-once.

     needsterminal
               This flag field indicates that the given shell command must be run on an interactive termi‐
               nal.  S-nail will temporarily release the terminal to the given command in interactive mode,
               in non-interactive mode this entry will be entirely ignored; this flag implies
               x-mailx-noquote.

     copiousoutput
               A flag field which indicates that the output of the view command will be an extended stream
               of textual output that can be (re)integrated into S-nail's normal visual display.  It is mu‐
               tually exclusive with needsterminal.

     textualnewlines
               A flag field which indicates that this type of data is line-oriented and that, if encoded in
               ‘base64’, all newlines should be converted to canonical form (CRLF) before encoding, and
               will be in that form after decoding.  (Currently unused.)

     nametemplate
               This field gives a filename format, in which ‘%s’ will be replaced by a random string, the
               joined combination of which will be used as the filename denoted by
               MAILX_FILENAME_TEMPORARY.  One could specify that a GIF file being passed to an image viewer
               should have a name ending in ‘.gif’ by using ‘nametemplate=%s.gif’.  Note that S-nail ig‐
               nores the name template unless that solely specifies a filename suffix that consists of
               (ASCII) alphabetic and numeric characters, the underscore and dot only.

     x11-bitmap
               Names a file, in X11 bitmap (xbm) format, which points to an appropriate icon to be used to
               visually denote the presence of this kind of data.  This field is not used by S-nail.

     description
               A textual description that describes this type of data.

     x-mailx-even-if-not-interactive
               An extension flag test field — by default handlers without copiousoutput are entirely ig‐
               nored in non-interactive mode, but if this flag is set then their use will be considered.
               It is an error if this flag is set for commands that use the flag needsterminal.

     x-mailx-noquote
               An extension flag field that indicates that even a copiousoutput view command shall not be
               used to generate message quotes (as it would be by default).

     x-mailx-async
               Extension flag field that denotes that the given view command shall be executed asyn‐
               chronously, without blocking S-nail.  Cannot be used in conjunction with needsterminal.

     x-mailx-test-once
               Extension flag which denotes whether the given test command shall be evaluated once only and
               the (boolean) result be cached.  This is handy if some global unchanging condition is to be
               queried, like “running under the X Window System”.

     x-mailx-tmpfile
               Extension flag field that requests creation of a zero-sized temporary file, the name of
               which is to be placed in the environment variable MAILX_FILENAME_TEMPORARY.  It is an error
               to use this flag with commands that include a ‘%s’ format.

     x-mailx-tmpfile-fill
               Normally the MIME part content is passed to the handler via standard input; if this flag is
               set then the data will instead be written into the implied x-mailx-tmpfile.  In order to
               cause deletion of the temporary file you will have to set x-mailx-tmpfile-unlink explicitly!
               It is an error to use this flag with commands that include a ‘%s’ format.

     x-mailx-tmpfile-unlink
               Extension flag field that requests that the temporary file shall be deleted automatically
               when the command loop is entered again at latest.  (Do not use this for asynchronous han‐
               dlers.)  It is an error to use this flag with commands that include a ‘%s’ format, or in
               conjunction with x-mailx-async, or without also setting x-mailx-tmpfile or
               x-mailx-tmpfile-fill.

     x-mailx-tmpfile-keep
               Using the string ‘%s’ implies the three tmpfile related flags above, but if you want, e.g.,
               x-mailx-async and deal with the temporary file yourself, you can add in this flag to force‐
               fully ignore x-mailx-tmpfile-unlink.

     The standard includes the possibility to define any number of additional entry fields, prefixed by
     ‘x-’.  Flag fields apply to the entire “Mailcap” entry — in some unusual cases, this may not be desir‐
     able, but differentiation can be accomplished via separate entries, taking advantage of the fact that
     subsequent entries are searched if an earlier one does not provide enough information.  E.g., if a
     view command needs to specify the needsterminal flag, but the compose command shall not, the following
     will help out the latter (with enabled debug or an increased verbose level S-nail will show informa‐
     tion about handler evaluation):

           application/postscript; ps-to-terminal %s; needsterminal
           application/postscript; ps-to-terminal %s; compose=idraw %s

     In fields any occurrence of the format string ‘%t’ will be replaced by the ‘TYPE/SUBTYPE’ specifica‐
     tion.  Named parameters from the ‘Content-type:’ field may be placed in the command execution line us‐
     ing ‘%{’ followed by the parameter name and a closing ‘}’ character.  The entire parameter should ap‐
     pear as a single command line argument, regardless of embedded spaces; thus:

           # Message
           Content-type:  multipart/mixed; boundary=42

           # Mailcap file
           multipart/*; /usr/local/bin/showmulti \
             %t %{boundary}  ;  composetyped  = /usr/local/bin/makemulti

           # Executed shell command
           /usr/local/bin/showmulti multipart/mixed 42

     Note that S-nail does not support handlers for multipart MIME parts as shown in this example (as of
     today).  S-nail does not support the additional formats ‘%n’ and ‘%F’.  An example file, also showing
     how to properly deal with the expansion of ‘%s’, which includes any quotes that are necessary to make
     it a valid shell argument by itself and thus will cause undesired behaviour when placed in additional
     user-provided quotes:

           # Comment line
           text/richtext; richtext %s; copiousoutput

           text/x-perl; perl -cWT %s

           application/pdf; \
             infile=%s\; \
               trap "rm -f ${infile}" EXIT\; \
               trap "exit 75" INT QUIT TERM\; \
               mupdf %s; \
             x-mailx-async; x-mailx-tmpfile-keep

           application/*; echo "This is \"%t\" but \
               is 50 \% Greek to me" \; < %s head -c 1024 | cat -vet; \
             copiousoutput; x-mailx-noquote

     Further reading: HTML mail and MIME attachments, The mime.types files, mimetype, MAILCAPS,
     mime-counter-evidence, pipe-TYPE/SUBTYPE, pipe-EXTENSION.

   The .netrc file
     The .netrc file contains user credentials for machine accounts.  The default location ~/.netrc may be
     overridden by the NETRC environment variable.  It is possible to load encrypted .netrc files by using
     an appropriate value in netrc-pipe.

     The file consists of space, tabulator or newline separated tokens.  S-nail implements a parser that
     supports a superset of the original BSD syntax, but users should nonetheless be aware of portability
     glitches of that file format, shall their .netrc be usable across multiple programs and platforms:

     •   BSD does not support single, but only double quotation marks, e.g., ‘password="pass with spaces"’.
     •   BSD (only?) supports escaping of single characters via a reverse solidus (e.g., a space can be es‐
         caped via ‘\ ’), in- as well as outside of a quoted string.
     •   BSD does not require a final quotation mark of the last user input token.
     •   The original BSD (Berknet) parser also supported a format which allowed tokens to be separated
         with commas – whereas at least Hewlett-Packard still seems to support this syntax, S-nail does
         not!
     •   As a non-portable extension some widely-used programs support shell-style comments: if an input
         line starts, after any amount of whitespace, with a number sign ‘#’, then the rest of the line is
         ignored.
     •   Whereas other programs may require that the .netrc file is accessible by only the user if it con‐
         tains a password token for any other login than “anonymous”, S-nail will always require these
         strict permissions.

     Of the following list of supported tokens S-nail only uses (and caches) machine, login and password.
     At runtime the command netrc can be used to control S-nail's .netrc cache.

     machine name
               The hostname of the entries' machine, lowercase-normalized by S-nail before use.  Any fur‐
               ther file content, until either end-of-file or the occurrence of another machine or a
               default first-class token is bound (only related) to the machine name.

               As an extension that should not be the cause of any worries S-nail supports a single wild‐
               card prefix for name:

                     machine *.example.com login USER password PASS
                     machine pop3.example.com login USER password PASS
                     machine smtp.example.com login USER password PASS

               which would match ‘xy.example.com’ as well as ‘pop3.example.com’, but neither ‘example.com’
               nor ‘local.smtp.example.com’.  Note that in the example neither ‘pop3.example.com’ nor
               ‘smtp.example.com’ will be matched by the wildcard, since the exact matches take precedence
               (it is however faster to specify it the other way around).

     default   This is the same as machine except that it is a fallback entry that is used shall none of
               the specified machines match; only one default token may be specified, and it must be the
               last first-class token.

     login name
               The user name on the remote machine.

     password string
               The user's password on the remote machine.

     account string
               Supply an additional account password.  This is merely for FTP purposes.

     macdef name
               Define a macro.  A macro is defined with the specified name; it is formed from all lines be‐
               ginning with the next line and continuing until a blank line is (consecutive newline charac‐
               ters are) encountered.  (Note that macdef entries cannot be utilized by multiple machines,
               too, but must be defined following the machine they are intended to be used with.)  If a
               macro named init exists, it is automatically run as the last step of the login process.
               This is merely for FTP purposes.

EXAMPLES
   An example configuration
           # This example assumes v15.0 compatibility mode
           set v15-compat

           # Request strict TLL transport layer security checks
           set tls-verify=strict

           # Where are the up-to-date TLS certificates?
           # (Since we manage up-to-date ones explicitly, do not use any,
           # possibly outdated, default certificates shipped with OpenSSL)
           #set tls-ca-dir=/etc/ssl/certs
           set tls-ca-file=/etc/ssl/certs/ca-certificates.crt
           set tls-ca-no-defaults
           #set tls-ca-flags=partial-chain
           wysh set smime-ca-file="${tls-ca-file}" \
             smime-ca-no-defaults #smime-ca-flags="${tls-ca-flags}"

           # This could be outsourced to a central configuration file via
           # tls-config-file plus tls-config-module if the used library allows.
           # CipherString: explicitly define the list of ciphers, which may
           #   improve security, especially with protocols older than TLS v1.2.
           #   See ciphers(1).  Possibly best to only use tls-config-pairs-HOST
           #   (or -USER@HOST), as necessary, again..
           #   Note that TLSv1.3 uses Ciphersuites= instead, which will join
           #   with CipherString (if protocols older than v1.3 are allowed)
           # Curves: especially with TLSv1.3 curves selection may be desired.
           # MinProtocol,MaxProtocol: do not use protocols older than TLS v1.2.
           #   Change this only when the remote server does not support it:
           #   maybe use chain support via tls-config-pairs-HOST / -USER@HOST
           #   to define such explicit exceptions, then, e.g.
           #     MinProtocol=TLSv1.1
           if [ "$tls-features" =% +ctx-set-maxmin-proto ]
             wysh set tls-config-pairs='\
                 CipherString=TLSv1.2:!aNULL:!eNULL:@STRENGTH,\
                 Curves=P-521:P-384:P-256,\
                 MinProtocol=TLSv1.1'
           else
             wysh set tls-config-pairs='\
                 CipherString=TLSv1.2:!aNULL:!eNULL:@STRENGTH,\
                 Curves=P-521:P-384:P-256,\
                 Protocol=-ALL\,+TLSv1.1 \, +TLSv1.2\, +TLSv1.3'
           endif

           # Essential setting: select allowed character sets
           set sendcharsets=utf-8,iso-8859-1

           # A very kind option: when replying to a message, first try to
           # use the same encoding that the original poster used herself!
           set reply-in-same-charset

           # When replying, do not merge From: and To: of the original message
           # into To:.  Instead old From: -> new To:, old To: -> merge Cc:.
           set recipients-in-cc

           # When sending messages, wait until the Mail-Transfer-Agent finishs.
           # Only like this you will be able to see errors reported through the
           # exit status of the MTA (including the built-in SMTP one)!
           set sendwait

           # Only use built-in MIME types, no mime.types(5) files
           set mimetypes-load-control

           # Default directory where we act in (relative to $HOME)
           set folder=mail
           # A leading "+" (often) means: under *folder*
           # *record* is used to save copies of sent messages
           set MBOX=+mbox.mbox DEAD=+dead.txt \
             record=+sent.mbox record-files record-resent

           # Make "file mymbox" and "file myrec" go to..
           shortcut mymbox %:+mbox.mbox myrec +sent.mbox

           # Not really optional, e.g., for S/MIME
           set from='Your Name <address@exam.ple>'

           # It may be necessary to set hostname and/or smtp-hostname
           # if the "SERVER" of mta and "domain" of from do not match.
           # The `urlencode' command can be used to encode USER and PASS
           set mta=(smtps?|submissions?)://[USER[:PASS]@]SERVER[:PORT] \
             smtp-auth=login/plain... \
             smtp-use-starttls

           # Never refuse to start into interactive mode, and more
           set emptystart \
             colour-pager crt= \
             followup-to followup-to-honour=ask-yes fullnames \
             history-file=+.s-nailhist history-size=-1 history-gabby \
             mime-counter-evidence=0b1111 \
             prompt='?\$?!\$!/\$^ERRNAME[\$account#\$mailbox-display]? ' \
             reply-to-honour=ask-yes \
             umask=

           # Only include the selected header fields when typing messages
           headerpick type retain from_ date from to cc subject \
             message-id mail-followup-to reply-to
           # ...when forwarding messages
           headerpick forward retain subject date from to cc
           # ...when saving message, etc.
           #headerpick save ignore ^Original-.*$ ^X-.*$

           # Some mailing lists
           mlist '@xyz-editor\.xyz$' '@xyzf\.xyz$'
           mlsubscribe '^xfans@xfans\.xyz$'

           # Handle a few file extensions (to store MBOX databases)
           filetype bz2 'bzip2 -dc' 'bzip2 -zc' \
             gz 'gzip -dc' 'gzip -c'  xz 'xz -dc' 'xz -zc' \
             zst 'zstd -dc' 'zstd -19 -zc' \
             zst.pgp 'gpg -d | zstd -dc' 'zstd -19 -zc | gpg -e'

           # A real life example of a very huge free mail provider
           # Instead of directly placing content inside `account',
           # we `define' a macro: like that we can switch "accounts"
           # from within *on-compose-splice*, for example!
           define XooglX {
             set folder=~/spool/XooglX inbox=+syste.mbox sent=+sent
             set from='Your Name <address@examp.ple>'

             set pop3-no-apop-pop.gmXil.com
             shortcut pop %:pop3s://pop.gmXil.com
             shortcut imap %:imaps://imap.gmXil.com
             # Or, entirely IMAP based setup
             #set folder=imaps://imap.gmail.com record="+[Gmail]/Sent Mail" \
             #   imap-cache=~/spool/cache

             set mta=smtp://USER:PASS@smtp.gmXil.com smtp-use-starttls
             # Alternatively:
             set mta=smtps://USER:PASS@smtp.gmail.com:465
           }
           account XooglX {
             \call XooglX
           }

           # Here is a pretty large one which does not allow sending mails
           # if there is a domain name mismatch on the SMTP protocol level,
           # which would bite us if the value of from does not match, e.g.,
           # for people who have a sXXXXeforge project and want to speak
           # with the mailing list under their project account (in from),
           # still sending the message through their normal mail provider
           define XandeX {
             set folder=~/spool/XandeX inbox=+syste.mbox sent=+sent
             set from='Your Name <address@exam.ple>'

             shortcut pop %:pop3s://pop.yaXXex.com
             shortcut imap %:imaps://imap.yaXXex.com

             set mta=smtps://USER:PASS@smtp.yaXXex.com:465 \
               hostname=yaXXex.com smtp-hostname=
           }
           account XandeX {
             \call Xandex
           }

           # Create some new commands so that, e.g., `ls /tmp' will..
           commandalias lls '!ls ${LS_COLOUR_FLAG} -aFlrS'
           commandalias llS '!ls ${LS_COLOUR_FLAG} -aFlS'

           set pipe-message/external-body='@* echo $MAILX_EXTERNAL_BODY_URL'

           # We do not support gpg(1) directly yet.  But simple --clearsign'd
           # message parts can be dealt with as follows:
           define V {
             localopts yes
             wysh set pipe-text/plain=$'@*#++=@\
               < "${MAILX_FILENAME_TEMPORARY}" awk \
                   -v TMPFILE="${MAILX_FILENAME_TEMPORARY}" \'\
                 BEGIN{done=0}\
                 /^-----BEGIN PGP SIGNED MESSAGE-----/,/^$/ {\
                   if(done++ != 0)\
                     next;\
                   print "--- GPG --verify ---";\
                   system("gpg --verify " TMPFILE " 2>&1");\
                   print "--- GPG --verify ---";\
                   print "";\
                   next;\
                 }\
                 /^-----BEGIN PGP SIGNATURE-----/,\
                     /^-----END PGP SIGNATURE-----/{\
                   next;\
                 }\
                 {print}\
               \''
               print
           }
           commandalias V '\'call V

     When storing passwords in ~/.mailrc appropriate permissions should be set on this file with ‘$ chmod
     0600 ~/.mailrc’.  If the [Option]al netrc-lookup is available user credentials can be stored in the
     central ~/.netrc file instead; e.g., here is a different version of the example account that sets up
     SMTP and POP3:

           define XandeX {
             set folder=~/spool/XandeX inbox=+syste.mbox sent=+sent
             set from='Your Name <address@exam.ple>'
             set netrc-lookup
             # Load an encrypted ~/.netrc by uncommenting the next line
             #set netrc-pipe='gpg -qd ~/.netrc.pgp'

             set mta=smtps://smtp.yXXXXx.ru:465 \
                 smtp-hostname= hostname=yXXXXx.com
             set pop3-keepalive=240 pop3-no-apop-pop.yXXXXx.ru
             commandalias xp fi pop3s://pop.yXXXXx.ru
           }
           account XandeX {
             \call XandeX
           }

     and, in the ~/.netrc file:

           machine *.yXXXXx.ru login USER password PASS

     This configuration should now work just fine:

           $ echo text | s-nail -dvv -AXandeX -s Subject user@exam.ple

   S/MIME step by step
     [Option] The first thing that is needed for Signed and encrypted messages with S/MIME is a personal
     certificate, and a private key.  The certificate contains public information, in particular a name and
     email address(es), and the public key that can be used by others to encrypt messages for the certifi‐
     cate holder (the owner of the private key), and to verify signed messages generated with that certifi‐
     cate('s private key).  Whereas the certificate is included in each signed message, the private key
     must be kept secret.  It is used to decrypt messages that were previously encrypted with the public
     key, and to sign messages.

     For personal use it is recommended that get a S/MIME certificate from one of the major CAs on the In‐
     ternet.  Many CAs offer such certificates for free.  Usually offered is a combined certificate and
     private key in PKCS#12 format which S-nail does not accept directly.  To convert it to PEM format, the
     following shell command can be used; please read on for how to use these PEM files.

           $ openssl pkcs12 -in cert.p12 -out certpem.pem -clcerts -nodes
           $ # Alternatively
           $ openssl pkcs12 -in cert.p12 -out cert.pem -clcerts -nokeys
           $ openssl pkcs12 -in cert.p12 -out key.pem -nocerts -nodes

     There is also https://www.CAcert.org which issues client and server certificates to members of their
     community for free; their root certificate (https://www.cacert.org/certs/root.crt) is often not in the
     default set of trusted CA root certificates, though, which means their root certificate has to be
     downloaded separately, and needs to be part of the S/MIME certificate validation chain by including it
     in smime-ca-dir or as a vivid member of the smime-ca-file.  But let us take a step-by-step tour on how
     to setup S/MIME with a certificate from CAcert.org despite this situation!

     First of all you will have to become a member of the CAcert.org community, simply by registrating
     yourself via the web interface.  Once you are, create and verify all email addresses you want to be
     able to create signed and encrypted messages for/with using the corresponding entries of the web in‐
     terface.  Now ready to create S/MIME certificates, so let us create a new “client certificate”, ensure
     to include all email addresses that should be covered by the certificate in the following web form,
     and also to use your name as the “common name”.

     Create a private key and a certificate request on your local computer (please see the manual pages of
     the used commands for more in-depth knowledge on what the used arguments etc. do):

           $ openssl req -nodes -newkey rsa:4096 -keyout key.pem -out creq.pem

     Afterwards copy-and-paste the content of “creq.pem” into the certificate-request (CSR) field of the
     web form on the CAcert.org website (you may need to unfold some “advanced options” to see the corre‐
     sponding text field).  This last step will ensure that your private key (which never left your box)
     and the certificate belong together (through the public key that will find its way into the certifi‐
     cate via the certificate-request).  You are now ready and can create your CAcert certified certifi‐
     cate.  Download and store or copy-and-paste it as “pub.crt”.

     Yay.  In order to use your new S/MIME setup a combined private key/public key (certificate) file has
     to be created:

           $ cat key.pem pub.crt > ME@HERE.com.paired

     This is the file S-nail will work with.  If you have created your private key with a passphrase then
     S-nail will ask you for it whenever a message is signed or decrypted, unless this operation has been
     automated as described in Signed and encrypted messages with S/MIME.  Set the following variables to
     henceforth use S/MIME (setting smime-ca-file is of interest for verification only):

           ? set smime-ca-file=ALL-TRUSTED-ROOT-CERTS-HERE \
               smime-sign-cert=ME@HERE.com.paired \
               smime-sign-digest=SHA512 \
               smime-sign

   Using CRLs with S/MIME or TLS
     [Option] Certification authorities (CAs) issue certificate revocation lists (CRLs) on a regular basis.
     These lists contain the serial numbers of certificates that have been declared invalid after they have
     been issued.  Such usually happens because the private key for the certificate has been compromised,
     because the owner of the certificate has left the organization that is mentioned in the certificate,
     etc.  To seriously use S/MIME or TLS verification, an up-to-date CRL is required for each trusted CA.
     There is otherwise no method to distinguish between valid and invalidated certificates.  S-nail cur‐
     rently offers no mechanism to fetch CRLs, nor to access them on the Internet, so they have to be re‐
     trieved by some external mechanism.

     S-nail accepts CRLs in PEM format only; CRLs in DER format must be converted, like, e.g.:

           $ openssl crl -inform DER -in crl.der -out crl.pem

     To tell S-nail about the CRLs, a directory that contains all CRL files (and no other files) must be
     created.  The smime-crl-dir or tls-crl-dir variables, respectively, must then be set to point to that
     directory.  After that, S-nail requires a CRL to be present for each CA that is used to verify a cer‐
     tificate.

FAQ
     In general it is a good idea to turn on debug (-d) and / or verbose (-v, twice) if something does not
     work well.  Very often a diagnostic message can be produced that leads to the problems' solution.

   S-nail shortly hangs on startup
     This can have two reasons, one is the necessity to wait for a file lock and cannot be helped, the
     other being that S-nail calls the function uname(2) in order to query the nodename of the box (some‐
     times the real one is needed instead of the one represented by the internal variable hostname).  One
     may have varying success by ensuring that the real hostname and ‘localhost’ have entries in
     /etc/hosts, or, more generally, that the name service is properly setup – and does hostname(1) return
     the expected value?  Does this local hostname have a domain suffix?  RFC 6762 standardized the link-
     local top-level domain ‘.local’, try again after adding an (additional) entry with this extension.

   I cannot login to Google mail aka GMail
     Since 2014 some free service providers classify programs as “less secure” unless they use a special
     authentication method (OAuth 2.0) which was not standardized for non-HTTP protocol authentication to‐
     ken query until August 2015 (RFC 7628).

     Different to Kerberos / GSSAPI, which is developed since the mid of the 1980s, where a user can easily
     create a local authentication ticket for her- and himself with the locally installed kinit(1) program,
     that protocol has no such local part but instead requires a world-wide-web query to create or fetch a
     token; since there is no local cache this query would have to be performed whenever S-nail is invoked
     (in interactive sessions situation may differ).

     S-nail does not support OAuth.  Because of this it is necessary to declare S-nail a “less secure app”
     (on the providers account web page) in order to read and send mail.  However, it also seems possible
     to take the following steps instead:

     1.   give the provider the number of a mobile phone,
     2.   enable “2-Step Verification”,
     3.   create an application specific password (16 characters), and
     4.   use that special password instead of the real Google account password in S-nail (for more on that
          see the section On URL syntax and credential lookup).

   Not "defunctional", but the editor key does not work
     It can happen that the terminal library (see On terminal control and line editor, bind, termcap) re‐
     ports different codes than the terminal really sends, in which case S-nail will tell that a key bind‐
     ing is functional, but will not be able to recognize it because the received data does not match any‐
     thing expected.  Especially without the [Option]al terminal capability library support one reason for
     this may be that the (possibly even non-existing) keypad is not turned on and the resulting layout re‐
     ports the keypad control codes for the normal keyboard keys.  The verbose listing of bindings will
     show the byte sequences that are expected.

     To overcome the situation, use, e.g., the program cat(1), in conjunction with the command line option
     -v, if available, to see the byte sequences which are actually produced by keypresses, and use the
     variable termcap to make S-nail aware of them.  E.g., the terminal this is typed on produces some
     false sequences, here an example showing the shifted home key:

           ? set verbose
           ? bind*
           # 1B 5B=[ 31=1 3B=; 32=2 48=H
             bind base :kHOM z0
           ? x
           $ cat -v
           ^[[H
           $ s-nail -v -Stermcap='kHOM=\E[H'
           ? bind*
           # 1B 5B=[ 48=H
             bind base :kHOM z0

   Can S-nail git-send-email?
     Yes.  Put (at least parts of) the following in your ~/.gitconfig:

           [sendemail]
           smtpserver = /usr/bin/s-mailx
           smtpserveroption = -t
           #smtpserveroption = -Sexpandaddr
           smtpserveroption = -Athe-account-you-need
           ##
           suppresscc = all
           suppressfrom = false
           assume8bitEncoding = UTF-8
           #to = /tmp/OUT
           confirm = always
           chainreplyto = true
           multiedit = false
           thread = true
           quiet = true
           annotate = true

IMAP CLIENT
     [Option]ally there is IMAP client support available.  This part of the program is obsolete and will
     vanish in v15 with the large MIME and I/O layer rewrite, because it uses old-style blocking I/O and
     makes excessive use of signal based long code jumps.  Support can hopefully be readded later based on
     a new-style I/O, with SysV signal handling.  In fact the IMAP support had already been removed from
     the codebase, but was reinstantiated on user demand: in effect the IMAP code is at the level of S-nail
     v14.8.16 (with imapcodec being the sole exception), and should be treated with some care.

     IMAP uses the ‘imap://’ and ‘imaps://’ protocol prefixes, and an IMAP-based folder may be used.  IMAP
     URLs (paths) undergo inspections and possible transformations before use (and the command imapcodec
     can be used to manually apply them to any given argument).  Hierarchy delimiters are normalized, a
     step which is configurable via the imap-delim variable chain, but defaults to the first seen delimiter
     otherwise.  S-nail supports internationalised IMAP names, and en- and decodes the names from and to
     the ttycharset as necessary and possible.  If a mailbox name is expanded (see Filename
     transformations) to an IMAP mailbox, all names that begin with `+' then refer to IMAP mailboxes below
     the folder target box, while folder names prefixed by `@' refer to folders below the hierarchy base,
     e.g., the following lists all folders below the current one when in an IMAP mailbox: ‘folders @’.

     Note: some IMAP servers do not accept the creation of mailboxes in the hierarchy base, but require
     that they are created as subfolders of `INBOX' – with such servers a folder name of the form

           imaps://mylogin@imap.myisp.example/INBOX.

     should be used (the last character is the server's hierarchy delimiter).  The following IMAP-specific
     commands exist:

     cache     Only applicable to cached IMAP mailboxes; takes a message list and reads the specified mes‐
               sages into the IMAP cache.

     connect   If operating in disconnected mode on an IMAP mailbox, switch to online mode and connect to
               the mail server while retaining the mailbox status.  See the description of the disconnected
               variable for more information.

     disconnect
               If operating in online mode on an IMAP mailbox, switch to disconnected mode while retaining
               the mailbox status.  See the description of the disconnected variable for more.  A list of
               messages may optionally be given as argument; the respective messages are then read into the
               cache before the connection is closed, thus ‘disco *’ makes the entire mailbox available for
               disconnected use.

     imap      Sends command strings directly to the current IMAP server.  S-nail operates always in IMAP
               `selected state' on the current mailbox; commands that change this will produce undesirable
               results and should be avoided.  Useful IMAP commands are:

                     create         Takes the name of an IMAP mailbox as an argument and creates it.

                     getquotaroot   (RFC 2087) Takes the name of an IMAP mailbox as an argument and prints
                                    the quotas that apply to the mailbox.  Not all IMAP servers support
                                    this command.

                     namespace      (RFC 2342) Takes no arguments and prints the Personal Namespaces, the
                                    Other User's Namespaces and the Shared Namespaces.  Each namespace type
                                    is printed in parentheses; if there are multiple namespaces of the same
                                    type, inner parentheses separate them.  For each namespace a prefix and
                                    a hierarchy separator is listed.  Not all IMAP servers support this
                                    command.

     imapcodec
               Perform IMAP path transformations.  Supports vput (see Command modifiers), and manages the
               error number !.  The first argument specifies the operation: e[ncode] normalizes hierarchy
               delimiters (see imap-delim) and converts the strings from the locale ttycharset to the in‐
               ternationalized variant used by IMAP, d[ecode] performs the reverse operation.

     The following IMAP-specific internal variables exist:

     disconnected
               (Boolean) When an IMAP mailbox is selected and this variable is set, no connection to the
               server is initiated.  Instead, data is obtained from the local cache (see imap-cache).
               Mailboxes that are not present in the cache and messages that have not yet entirely been
               fetched from the server are not available; to fetch all messages in a mailbox at once, the
               command `copy * /dev/null' can be used while still in connected mode.  Changes that are made
               to IMAP mailboxes in disconnected mode are queued and committed later when a connection to
               that server is made.  This procedure is not completely reliable since it cannot be guaran‐
               teed that the IMAP unique identifiers (UIDs) on the server still match the ones in the cache
               at that time.  Data is saved to DEAD when this problem occurs.

     disconnected-USER@HOST
               The specified account is handled as described for the disconnected variable above, but other
               accounts are not affected.

     imap-auth-USER@HOST, imap-auth
               Sets the IMAP authentication method.  Valid values are `login' for the usual password-based
               authentication (the default), `cram-md5', which is a password-based authentication that does
               not send the password over the network in clear text, and `gssapi' for GSS-API based authen‐
               tication.

     imap-cache
               Enables caching of IMAP mailboxes.  The value of this variable must point to a directory
               that is either existent or can be created by S-nail.  All contents of the cache can be
               deleted by S-nail at any time; it is not safe to make assumptions about them.

     imap-delim-USER@HOST, imap-delim-HOST, imap-delim
               The hierarchy separator used by the IMAP server.  Whenever an IMAP path is specified it will
               undergo normalization.  One of the normalization steps is the squeezing and adjustment of
               hierarchy separators.  If this variable is set, any occurrence of any character of the given
               value that exists in the path will be replaced by the first member of the value; an empty
               value will cause the default to be used, it is ‘/.’.  If not set, we will reuse the first
               hierarchy separator character that is discovered in a user-given mailbox name.

     imap-keepalive-USER@HOST, imap-keepalive-HOST, imap-keepalive
               IMAP servers may close the connection after a period of inactivity; the standard requires
               this to be at least 30 minutes, but practical experience may vary.  Setting this variable to
               a numeric `value' greater than 0 causes a `NOOP' command to be sent each `value' seconds if
               no other operation is performed.

     imap-list-depth
               When retrieving the list of folders on an IMAP server, the folders command stops after it
               has reached a certain depth to avoid possible infinite loops.  The value of this variable
               sets the maximum depth allowed.  The default is 2.  If the folder separator on the current
               IMAP server is a slash `/', this variable has no effect and the folders command does not de‐
               scend to subfolders.

     imap-use-starttls-USER@HOST, imap-use-starttls-HOST, imap-use-starttls
               Causes S-nail to issue a `STARTTLS' command to make an unencrypted IMAP session TLS en‐
               crypted.  This functionality is not supported by all servers, and is not used if the session
               is already encrypted by the IMAPS method.

SEE ALSO
     bogofilter(1), gpg(1), more(1), newaliases(1), openssl(1), sendmail(1), sh(1), spamassassin(1),
     iconv(3), setlocale(3), aliases(5), termcap(5), terminfo(5), locale(7), mailaddr(7), re_format(7),
     mailwrapper(8), sendmail(8)

HISTORY
     M. Douglas McIlroy writes in his article “A Research UNIX Reader: Annotated Excerpts from the
     Programmer's Manual, 1971-1986” that a mail(1) command already appeared in First Edition UNIX in 1971:

           Electronic mail was there from the start.  Never satisfied with its exact behavior, everybody
           touched it at one time or another: to assure the safety of simultaneous access, to improve pri‐
           vacy, to survive crashes, to exploit uucp, to screen out foreign freeloaders, or whatever.  Not
           until v7 did the interface change (Thompson).  Later, as mail became global in its reach, Dave
           Presotto took charge and brought order to communications with a grab-bag of external networks
           (v8).

     BSD Mail was written in 1978 by Kurt Shoens and developed as part of the BSD UNIX distribution until
     1995.  Mail has then seen further development in open source BSD variants, noticeably by Christos
     Zoulas in NetBSD.  Based upon this Nail, later Heirloom Mailx, was developed by Gunnar Ritter in the
     years 2000 until 2008.  Since 2012 S-nail is maintained by Steffen (Daode) Nurpmeso.  This man page is
     derived from “The Mail Reference Manual” that was originally written by Kurt Shoens.

AUTHORS
     Kurt Shoens, Edward Wang, Keith Bostic, Christos Zoulas, Gunnar Ritter.  S-nail is developed by
     Steffen Nurpmeso <s-mailx@lists.sdaoden.eu>.

CAVEATS
     [v15 behaviour may differ] Interrupting an operation via SIGINT aka ‘control-C’ from anywhere else but
     a command prompt is very problematic and likely to leave the program in an undefined state: many li‐
     brary functions cannot deal with the siglongjmp(3) that this software (still) performs; even though
     efforts have been taken to address this, no sooner but in v15 it will have been worked out: interrup‐
     tions have not been disabled in order to allow forceful breakage of hanging network connections, for
     example (all this is unrelated to ignore).

     The SMTP and POP3 protocol support of S-nail is very basic.  Also, if it fails to contact its upstream
     SMTP server, it will not make further attempts to transfer the message at a later time (setting save
     and sendwait may be useful).  If this is a concern, it might be better to set up a local SMTP server
     that is capable of message queuing.

BUGS
     After deleting some message of a POP3 mailbox the header summary falsely claims that there are no mes‐
     sages to display, one needs to perform a scroll or dot movement to restore proper state.  In
     ‘thread’ed sort mode a power user may encounter crashes very occasionally (this is may and very).  The
     file TODO in the source repository lists future directions.

     Please report bugs to the contact-mail address, e.g., from within s-nail: ‘? eval mail $contact-mail’.
     Including the verbose output of the command version may be helpful, e.g.,

           ? wysh set escape=! verbose; vput version xy; unset verbose;\
             eval mail $contact-mail
           Bug subject
           !I xy
           !.

     Information on the web at ‘$ s-nail -X 'echo $contact-web' -Xx’.

BSD                                           August 08, 2018                                           BSD

s-nail (s-nail v14.9.11): send and receive Internet mail

Send-only mode: send mail "to-address" receiver(s):
  s-nail [-DdEFinv~#] [-: spec] [-A account] [:-C "custom: header":]
         [:-a attachment:] [:-b bcc-address:] [:-c cc-address:]
         [-M type | -m file | -q file | -t] [-r from-address]
         [:-S var[=value]:] [-s subject] [:-X cmd:] [-.] :to-address:

"Receive" mode, starting on [-u user], primary *inbox* or [$MAIL]:
  s-nail [-DdEeHiNnRv~#] [-: spec] [-A account] [:-C "custom: header":]
         [-L spec] [-r from-address] [:-S var[=value]:] [-u user] [:-X cmd:]

"Receive" mode, starting on -f (secondary $MBOX or [file]):
  s-nail [-DdEeHiNnRv~#] [-: spec] [-A account] [:-C "custom: header":] -f
         [-L spec] [-r from-address] [:-S var[=value]:] [:-X cmd:] [file]

. -d sandbox, -:/ no .rc files, -. end options and force send-mode
. -a attachment[=input-charset[#output-charset]]
. -b, -c, to-address, (-r): ex@am.ple or '(Lovely) Ex <am@p.le>'
. -[Mmqt]: special input data (-t: template message on stdin)
. -e only mail check, -H header summary; both: message specification via -L
. -S (un)sets variable, -X executes command(s), -# enters batch mode
. Features via "$ s-nail -Xversion -Xx"
. Bugs/Contact via "$ s-nail -Sexpandaddr=shquote '\$contact-mail'"