emacs-gpu/doc/emacs/sending.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985--1987, 1993--1995, 1997, 2000--2026 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Sending Mail
@chapter Sending Mail
@cindex sending mail
@cindex mail
@cindex email
@cindex message

@kindex C-x m
@findex compose-mail
  To send an email message from Emacs, type @kbd{C-x m}.  This
switches to a buffer named @file{*unsent mail*}, where you can edit
the text and headers of the message.  When done, type @kbd{C-c C-s} or
@kbd{C-c C-c} to send it.

@table @kbd
@item C-x m
Begin composing mail (@code{compose-mail}).
@item C-x 4 m
Likewise, in another window (@code{compose-mail-other-window}).
@item C-x 5 m
Likewise, but in a new frame (@code{compose-mail-other-frame}).
@item C-c C-s
In the mail buffer, send the message (@code{message-send}).
@item C-c C-c
In the mail buffer, send the message and bury the buffer
(@code{message-send-and-exit}).
@end table

  The mail buffer is an ordinary Emacs buffer, so you can switch to
other buffers while composing the mail.  If you want to send another
message before finishing the current one, type @kbd{C-x m} again to
open a new mail buffer whose name has a different numeric suffix
(@pxref{Misc Buffer}).  (This only works if you use the default
Message mode to compose email; see @ref{Mail Commands}.)  If you know
that you'd like to continue composing the unsent message you were
editing, invoke this command with a prefix argument, @w{@kbd{C-u C-x
m}}, and Emacs will switch to the last mail buffer you used and let
you pick up editing the message where you left off.

@kindex C-x 4 m
@findex compose-mail-other-window
@kindex C-x 5 m
@findex compose-mail-other-frame
  The command @kbd{C-x 4 m} (@code{compose-mail-other-window}) does
the same as @kbd{C-x m}, except it displays the mail buffer in a
different window.  The command @kbd{C-x 5 m}
(@code{compose-mail-other-frame}) does it in a new frame.  @xref{Window
Choice}.

  When you type @kbd{C-c C-c} or @kbd{C-c C-s} to send the mail, Emacs
may ask you how it should deliver the mail---either directly via SMTP,
or using some other method.  @xref{Mail Sending}, for details.

@menu
* Format: Mail Format.        Format of a mail message.
* Headers: Mail Headers.      Details of some standard mail header fields.
* Aliases: Mail Aliases.      Abbreviating and grouping mail addresses.
* Commands: Mail Commands.    Special commands for editing mail being composed.
* Signature: Mail Signature.  Adding a signature to every message.
* Amuse: Mail Amusements.     Distracting the NSA; adding fortune messages.
* Methods: Mail Methods.      Using alternative mail-composition methods.
@end menu

@node Mail Format
@section The Format of the Mail Buffer

  Here is an example of the contents of a mail buffer:

@example
To: subotai@@example.org
CC: mongol.soldier@@example.net, rms@@gnu.org
Subject: Re: What is best in life?
From: conan@@example.org
--text follows this line--
To crush your enemies, see them driven before you, and to
hear the lamentation of their women.
@end example

@noindent
At the top of the mail buffer is a set of @dfn{header fields}, which
are used for specifying information about the email's recipient(s),
subject, and so on.  The above buffer contains header fields for
@samp{To}, @samp{CC}, @samp{Subject}, and @samp{From}.  Some header
fields are automatically pre-initialized in the mail buffer, when
appropriate.

  The line that says @samp{--text follows this line--} separates the
header fields from the @dfn{body} (or @dfn{text}) of the message.
Everything above that line is treated as part of the headers;
everything below it is treated as the body.  The delimiter line itself
does not appear in the message actually sent.

  You can insert and edit header fields using ordinary editing
commands.  @xref{Header Editing}, for commands specific to editing
header fields.  Certain headers, such as @samp{Date} and
@samp{Message-Id}, are normally omitted from the mail buffer and are
created automatically when the message is sent.

@node Mail Headers
@section Mail Header Fields
@cindex headers (of mail message)

  A header field in the mail buffer starts with a field name at the
beginning of a line, terminated by a colon.  Upper and lower case are
equivalent in field names.  After the colon and optional whitespace
comes the contents of the field.

  You can use any name you like for a header field, but normally
people use only standard field names with accepted meanings.

@vindex user-full-name
@vindex user-mail-address
  The @samp{From} header field identifies the person sending the email
(i.e., you).  This should be a valid mailing address, as replies are
normally sent there.  The default contents of this header field are
computed from the variables @code{user-full-name} (which specifies
your full name) and @code{user-mail-address} (your email address).  On
some operating systems, Emacs initializes these two variables using
environment variables (@pxref{General Variables}).  If this
information is unavailable or wrong, you should customize the
variables yourself (@pxref{Easy Customization}).

  Apart from @samp{From}, here is a table of commonly-used fields:

@table @samp
@item To
The mailing address(es) to which the message is addressed.  To list
more than one address, use commas to separate them.

@item Subject
The subject of the message.

@item CC
Additional mailing address(es) to send the message to.  This is like
@samp{To}, except that these readers should not regard the message as
directed at them.

@item BCC
Additional mailing address(es) to send the message to, which should
not appear in the header of the message actually sent.  @samp{BCC} stands
for @dfn{blind carbon copies}.

@item FCC
The name of a file, to which a copy of the sent message should be
appended.  Emacs writes the message in mbox format, unless the file is
in Babyl format (used by Rmail before Emacs 23), in which case Emacs
writes in Babyl format.  If an Rmail buffer is visiting the file,
Emacs updates it accordingly.  To specify more than one file, use
several @samp{FCC} fields, with one file name in each field.

@item Reply-To
An address to which replies should be sent, instead of @samp{From}.
This is used if, for some reason, your @samp{From} address cannot
receive replies.

@item Mail-Reply-To
This field takes precedence over @samp{Reply-To}.  It is used because
some mailing lists set the @samp{Reply-To} field for their own
purposes (a somewhat controversial practice).

@item Mail-Followup-To
One of more address(es) to use as default recipient(s) for follow-up
messages.  This is typically used when you reply to a message from a
mailing list that you are subscribed to, and want replies to go to the
list without sending an extra copy to you.

@item In-Reply-To
An identifier for the message you are replying to.  Most mail readers
use this information to group related messages together.  Normally,
this header is filled in automatically when you reply to a message in
any mail program built into Emacs.

@item References
Identifiers for previous related messages.  Like @samp{In-Reply-To},
this is normally filled in automatically for you.
@end table

@noindent
The @samp{To}, @samp{CC}, and @samp{BCC} fields can appear any number
of times, and each such header field can contain multiple addresses,
separated by commas.  This way, you can specify any number of places
to send the message.  These fields can also have continuation lines:
one or more lines starting with whitespace, following the starting
line of the field, are considered part of the field.  Here's an
example of a @samp{To} field with a continuation line:

@example
@group
To: foo@@example.net, this@@example.net,
  bob@@example.com
@end group
@end example

@c There is also mail-specify-envelope-from and mail-envelope-from, but
@c these are probably not topics for the Emacs manual.

@vindex mail-default-headers
  You can direct Emacs to insert certain default headers into the mail
buffer by setting the variable @code{mail-default-headers} to a
string.  Then @kbd{C-x m} inserts this string into the message
headers.  For example, here is how to add a @samp{Reply-To} and
@samp{FCC} header to each message:

@smallexample
(setq mail-default-headers
      "Reply-To: foo@@example.com\nFCC: ~/Mail/sent")
@end smallexample

@noindent
If the default header fields are not appropriate for a
particular message, edit them as necessary before sending the message.

@node Mail Aliases
@section Mail Aliases
@cindex mail aliases
@cindex @file{.mailrc} file
@cindex mailrc file
@vindex mail-personal-alias-file

  You can define @dfn{mail aliases}, which are short mnemonic names
that stand for one or more mailing addresses.  By default, mail
aliases are defined in the file @file{~/.mailrc}.  You can specify a
different file name to use, by setting the variable
@code{mail-personal-alias-file}.

  To define an alias in @file{~/.mailrc}, write a line like this:

@example
alias @var{nick} @var{fulladdresses}
@end example

@noindent
This means that @var{nick} should expand into @var{fulladdresses},
where @var{fulladdresses} can be either a single address, or multiple
addresses separated with spaces.  For instance, to make @code{maingnu}
stand for @code{gnu@@gnu.org} plus a local address of your own, put in
this line:

@example
alias maingnu gnu@@gnu.org local-gnu
@end example

@noindent
If an address contains a space, quote the whole address with a pair of
double quotes, like this:

@example
alias jsmith "John Q. Smith <none@@example.com>"
@end example

@noindent
Note that you need not include double quotes around individual parts
of the address, such as the person's full name.  Emacs puts them in if
they are needed.  For instance, it inserts the above address as
@samp{"John Q. Smith" <none@@example.com>}.

@findex rebuild-mail-abbrevs
@findex merge-mail-abbrevs
  After editing the @file{~/.mailrc} file, or if the file was modified
outside of Emacs, you can update the mail aliases used by a running
Emacs session with @w{@kbd{M-x rebuild-mail-abbrevs @key{RET}}}.  This
prompts for the name of the file to use, the default being the value
of @code{mail-personal-alias-file}.  A similar command
@code{merge-mail-abbrevs} prompts for a file with mail aliases, then
merges the aliases in that file with the existing ones.

@findex define-mail-abbrev
  Alternatively, you can use Emacs commands to define mail aliases.
The command @code{define-mail-abbrev} prompts for the alias and the
full address, and defines the alias to expand to the full address.
Emacs will save the added aliases whenever it offers to save all
files (for @kbd{C-x s} or @kbd{C-x C-c}), like it does with other
abbrevs (@pxref{Saving Abbrevs}).

  Emacs also recognizes include commands in @file{~/.mailrc}.  They
look like this:

@example
source @var{filename}
@end example

@noindent
The @file{~/.mailrc} file is not unique to Emacs; many other
mail-reading programs use it for mail aliases, and it can contain
various other commands.  However, Emacs ignores everything except
alias definitions and include commands.

@findex mail-abbrev-insert-alias
  Mail aliases expand as abbrevs---that is to say, as soon as you type
a word-separator character after an alias (@pxref{Abbrevs}).  This
expansion takes place only within the @samp{To}, @samp{From},
@samp{CC}, @samp{BCC}, and @samp{Reply-To} header fields (plus their
@samp{Resent-} variants); it does not take place in other header
fields, such as @samp{Subject}.

  You can also insert an aliased address directly, using the command
@kbd{M-x mail-abbrev-insert-alias}.  This reads an alias name, with
completion, and inserts its definition at point.

@findex mail-abbrev-complete-alias
  The command @code{mail-abbrev-complete-alias} completes on the mail
alias preceding point.

@node Mail Commands
@section Mail Commands
@cindex Message mode
@cindex mode, Message

  The default major mode for the @file{*mail*} buffer is called
Message mode.  It behaves like Text mode in many ways, but provides
several additional commands on the @kbd{C-c} prefix, which make
editing a message more convenient.

  In this section, we will describe some of the most commonly-used
commands available in Message mode.
@ifnottex
Message mode also has its own manual, where its features are described
in greater detail.  @xref{Top,,Message, message, Message}.
@end ifnottex

@menu
* Mail Sending::        Commands to send the message.
* Header Editing::      Commands to move to header fields and edit them.
* Citing Mail::         Quoting a message you are replying to.
* Mail Misc::           Attachments, spell checking, etc.
@end menu

@node Mail Sending
@subsection Mail Sending

@table @kbd
@item C-c C-c
Send the message, and bury the mail buffer (@code{message-send-and-exit}).
@item C-c C-s
Send the message, and leave the mail buffer selected (@code{message-send}).
@end table

@kindex C-c C-s @r{(Message mode)}
@kindex C-c C-c @r{(Message mode)}
@findex message-send
@findex message-send-and-exit
@vindex message-kill-buffer-on-exit
  The usual command to send a message is @kbd{C-c C-c}
(@code{message-send-and-exit}).  This sends the message and then
buries the mail buffer, putting it at the lowest priority for
reselection.  If you want it to kill the mail buffer instead, change
the variable @code{message-kill-buffer-on-exit} to @code{t}.

  The command @kbd{C-c C-s} (@code{message-send}) sends the message
and leaves the buffer selected.  Use this command if you want to
modify the message (perhaps with new recipients) and send it again.

@vindex message-send-hook
  Sending a message runs the hook @code{message-send-hook}.  It also
marks the mail buffer as unmodified, except if the mail buffer is also
a file-visiting buffer (in that case, only saving the file does that,
and you don't get a warning if you try to send the same message
twice).

@cindex SMTP
@cindex Feedmail
@cindex Sendmail
@cindex Mailclient
@vindex message-send-mail-function
@vindex send-mail-function
  The variable @code{message-send-mail-function} controls how the
message is delivered (@code{send-mail-function} is used for Mail mode).
The value of @code{send-mail-function} should be one of the following
functions:

@table @code
@item sendmail-query-once
Query for a delivery method (one of the other entries in this list),
and use that method for this message; then save the method to
@code{send-mail-function}, so that it is used for future deliveries.
This is the default, unless you have already set the variables for
sending mail via @code{smtpmail-send-it} (see below).

@item smtpmail-send-it
Send mail through an external mail host, such as your
Internet service provider's outgoing SMTP mail server.  If you have
not told Emacs how to contact the SMTP server, it prompts for this
information, which is saved in the @code{smtpmail-smtp-server} variable
and the file @file{~/.authinfo}.
@xref{Top,,Emacs SMTP Library, smtpmail, Sending mail via SMTP}.

@item sendmail-send-it
Send mail using the system's default @command{sendmail} program, or
equivalent.  This requires the system to be set up for delivering mail
directly via SMTP.

@item mailclient-send-it
Pass the mail buffer on to the system's designated mail client.  See
the commentary section in the file @file{mailclient.el} for details.

@item feedmail-send-it
This is similar to @code{sendmail-send-it}, but allows you to queue
messages for later sending.  See the commentary section in the file
@file{feedmail.el} for details.
@end table

  When you send a message containing non-@acronym{ASCII} characters,
they need to be encoded with a coding system (@pxref{Coding Systems}).
Usually the coding system is specified automatically by your chosen
language environment (@pxref{Language Environments}).  You can
explicitly specify the coding system for outgoing mail by setting the
variable @code{sendmail-coding-system} (@pxref{Recognize Coding}).  If
the coding system thus determined does not handle the characters in a
particular message, Emacs asks you to select the coding system to use,
showing a list of possible coding systems.  @xref{Output Coding}.

@node Header Editing
@subsection Mail Header Editing

  Message mode provides the following special commands to move to
particular header fields and to complete addresses in headers.

@table @kbd
@item C-c C-f C-t
Move to the @samp{To} header (@code{message-goto-to}).
@item C-c C-f C-s
Move to the @samp{Subject} header (@code{message-goto-subject}).
@item C-c C-f C-c
Move to the @samp{CC} header (@code{message-goto-cc}).
@item C-c C-f C-b
Move to the @samp{BCC} header (@code{message-goto-bcc}).
@item C-c C-f C-r
Move to the @samp{Reply-To} header (@code{message-goto-reply-to}).
@item C-c C-f C-f
Move to the @samp{Mail-Followup-To} header field
(@code{message-goto-followup-to}).
@item C-c C-f C-w
Add a new @samp{FCC} header field, with file-name completion
(@code{message-goto-fcc}).
@item C-c C-b
Move to the start of the message body (@code{message-goto-body}).
@item @key{TAB}
Complete a mailing address (@code{message-tab}).
@end table

@kindex C-c C-f C-t @r{(Message mode)}
@findex message-goto-to
@kindex C-c C-f C-s @r{(Message mode)}
@findex message-goto-subject
@kindex C-c C-f C-c @r{(Message mode)}
@findex message-goto-cc
@kindex C-c C-f C-b @r{(Message mode)}
@findex message-goto-bcc
@kindex C-c C-f C-r @r{(Message mode)}
@findex goto-reply-to
@kindex C-c C-f C-f @r{(Message mode)}
@findex goto-followup-to
@kindex C-c C-f C-w @r{(Message mode)}
@findex message-goto-fcc
  The commands to move point to particular header fields are all based
on the prefix @kbd{C-c C-f} (@samp{C-f} is for ``field'').  If the
field in question does not exist, the command creates one (the
exception is @code{mail-fcc}, which creates a new field each time).

@kindex C-c C-b @r{(Message mode)}
@findex mail-text
  The command @kbd{C-c C-b} (@code{message-goto-body}) moves point to
just after the header separator line---that is, to the beginning of
the body.

@findex message-tab
@kindex TAB @r{(Message mode)}
  While editing a header field that contains addresses, such as
@samp{To:}, @samp{CC:} and @samp{BCC:}, you can complete an address by
typing @key{TAB} (@code{message-tab}).  This attempts to insert the
full name corresponding to the address based on a couple of methods,
including EUDC, a library that recognizes a number of directory server
protocols (@pxref{Top,,EUDC,eudc, The Emacs Unified Directory
Client}).  Failing that, it attempts to expand the address as a mail
alias (@pxref{Mail Aliases}).  If point is on a header field that does
not take addresses, or if it is in the message body, then @key{TAB}
just inserts a tab character.

@node Citing Mail
@subsection Citing Mail
@cindex citing mail

@table @kbd
@item C-c C-y
Yank the selected message from the mail reader, as a citation
(@code{message-yank-original}).
@item C-c C-q
Fill each paragraph cited from another message
(@code{message-fill-yanked-message}).
@end table

@kindex C-c C-y @r{(Message mode)}
@findex message-yank-original
@findex message-yank-prefix
  You can use the command @kbd{C-c C-y} (@code{message-yank-original})
to @dfn{cite} a message that you are replying to.  This inserts the
text of that message into the mail buffer.  This command works only if
the mail buffer is invoked from a mail reader running in Emacs, such
as Rmail.

  By default, Emacs inserts the string @samp{>} in front of each line
of the cited text; this prefix string is specified by the variable
@code{message-yank-prefix}.  If you call @code{message-yank-original}
with a prefix argument, the citation prefix is not inserted.

@kindex C-c C-q @r{(Message mode)}
@findex mail-fill-yanked-message
  After using @kbd{C-c C-y}, you can type @kbd{C-c C-q}
(@code{message-fill-yanked-message}) to fill the paragraphs of the
cited message.  One use of @kbd{C-c C-q} fills all such paragraphs,
each one individually.  To fill a single paragraph of the quoted
message, use @kbd{M-q}.  If filling does not automatically handle the
type of citation prefix you use, try setting the fill prefix
explicitly.  @xref{Filling}.

@vindex mail-citation-hook
  You can customize mail citation through the hook
@code{mail-citation-hook}.  For example, you can use the Supercite
package, which provides more flexible citation
(@pxref{Introduction,,,sc, Supercite}).

@node Mail Misc
@subsection Mail Miscellany

@kindex C-c C-a @r{(Message mode)}
@findex mml-attach-file
@cindex MIME
@cindex Multipurpose Internet Mail Extensions
  You can @dfn{attach} a file to an outgoing message by typing
@kbd{C-c C-a} (@code{mml-attach-file}) in the mail buffer.  Attaching
is done using the Multipurpose Internet Mail Extensions
(@acronym{MIME}) standard.

  The @code{mml-attach-file} command prompts for the name of the file,
and for the attachment's @dfn{content type}, @dfn{description}, and
@dfn{disposition}.  The content type is normally detected
automatically; just type @key{RET} to accept the default.  The
description is a single line of text that the recipient will see next
to the attachment; you may also choose to leave this empty.  The
disposition is either @samp{inline}, which means the recipient will
see a link to the attachment within the message body, or
@samp{attachment}, which means the link will be separate from the
body.

@findex mail-add-attachment
  The @code{mml-attach-file} command is specific to Message mode; in
Mail mode use @kbd{mail-add-attachment} instead.  It will prompt only
for the name of the file, and will determine the content type and the
disposition automatically.  If you want to include some description of
the attached file, type that in the message body.

  The actual contents of the attached file are not inserted into the
mail buffer.  Instead, some placeholder text is inserted into the mail
buffer, like this:

@smallexample
<#part type="text/plain" filename="~/foo.txt" disposition=inline>
<#/part>
@end smallexample

@noindent
When you type @kbd{C-c C-c} or @kbd{C-c C-s} to send the message, the
attached file will be delivered with it.

@findex ispell-message
  While composing a message, you can do spelling correction on the
message text by typing @kbd{M-x ispell-message}.  If you have yanked
an incoming message into the outgoing draft, this command skips what
was yanked, but it checks the text that you yourself inserted (it
looks for indentation or @code{mail-yank-prefix} to distinguish the
cited lines from your input).  @xref{Spelling}.

@vindex message-mode-hook
@vindex message-setup-hook
  Turning on Message mode (which @kbd{C-x m} does automatically) runs
the normal hooks @code{text-mode-hook} and @code{message-mode-hook}.
Initializing a new outgoing message runs the normal hook
@code{message-setup-hook}; you can use this hook if you want to make
changes to the appearance of the mail buffer.  @xref{Hooks}.

  The main difference between these hooks is just when they are
invoked.  Whenever you type @kbd{C-x m}, @code{message-mode-hook} runs
as soon as the mail buffer is created.  Then the @code{message-setup}
function inserts the default contents of the buffer.  After these
default contents are inserted, @code{message-setup-hook} runs.

  If you use @kbd{C-x m} to continue an existing composition,
@code{message-mode-hook} runs immediately after switching to the mail
buffer.  If the buffer is unmodified, or if you decide to erase it and
start again, @code{message-setup-hook} runs after the default contents
are inserted.

@node Mail Signature
@section Mail Signature

@cindex mail signature
@vindex message-signature-file
@vindex message-signature
  You can add a standard piece of text---your @dfn{mail
signature}---to the end of every message.  This signature may contain
information such as your telephone number or your physical location.
The variable @code{message-signature} determines how Emacs handles the
mail signature.

  The default value of @code{message-signature} is @code{t}; this
means to look for your mail signature in the file @file{~/.signature}.
If this file exists, its contents are automatically inserted into the
end of the mail buffer.  You can change the signature file via the
variable @code{message-signature-file}.

  If you change @code{message-signature} to a string, that specifies
the text of the signature directly.

@kindex C-c C-w @r{(Message mode)}
@findex message-insert-signature
  If you change @code{message-signature} to @code{nil}, Emacs will not
insert your mail signature automatically.  You can insert your mail
signature by typing @kbd{C-c C-w} (@code{message-insert-signature}) in
the mail buffer.  Emacs will look for your signature in the signature
file.

@vindex mail-signature-file
@vindex mail-signature
  If you use Mail mode rather than Message mode for composing your
mail, the corresponding variables that determine how your signature is
sent are @code{mail-signature} and @code{mail-signature-file} instead.

  By convention, a mail signature should be marked by a line whose
contents are @samp{-- }.  If your signature lacks this prefix, it is
added for you.  The remainder of your signature should be no more than
four lines.

@node Mail Amusements
@section Mail Amusements

@findex spook
@cindex NSA
  @kbd{M-x spook} adds a line of randomly chosen keywords to an outgoing
mail message.  The keywords are chosen from a list of words that suggest
you are discussing something subversive.

  The idea behind this feature is the suspicion that the
NSA@footnote{The US National Security Agency.} and other intelligence
agencies snoop on all electronic mail messages that contain keywords
suggesting they might find them interesting.  (The agencies say that
they don't, but that's what they @emph{would} say.)  The idea is that if
lots of people add suspicious words to their messages, the agencies will
get so busy with spurious input that they will have to give up reading
it all.  Whether or not this is true, it at least amuses some people.

@findex fortune-to-signature
@cindex fortune cookies
  You can use the @code{fortune} program to put a fortune cookie
message into outgoing mail.  To do this, add
@code{fortune-to-signature} to @code{mail-setup-hook}:

@example
(add-hook 'mail-setup-hook 'fortune-to-signature)
@end example

@noindent
You will probably need to set the variable @code{fortune-file} before
using this.

@node Mail Methods
@section Mail-Composition Methods
@cindex mail-composition methods
@cindex Mail mode
@cindex mode, Mail

@cindex MH mail interface
@cindex Message mode for sending mail
  In this chapter we have described the usual Emacs mode for editing
and sending mail---Message mode.  This is only one of several
available modes.  Prior to Emacs 23.2, the default mode was Mail mode,
which is similar to Message mode in many respects but is less
feature-rich; for example, it supports only basic MIME: it allows you
to add attachments, but lacks more sophisticated MIME features.
Another available mode is MH-E (@pxref{Top,,MH-E,mh-e, The Emacs
Interface to MH}).

@vindex mail-user-agent
@findex define-mail-user-agent
  You can choose any of these @dfn{mail user agents} as your preferred
method for editing and sending mail.  The commands @kbd{C-x m},
@kbd{C-x 4 m} and @kbd{C-x 5 m} use whichever agent you have
specified; so do various other parts of Emacs that send mail, such as
the bug reporter (@pxref{Bugs}).  To specify a mail user agent,
customize the variable @code{mail-user-agent}.  Currently, legitimate
values include @code{message-user-agent} (Message mode)
@code{sendmail-user-agent} (Mail mode), @code{gnus-user-agent}, and
@code{mh-e-user-agent}.  Additional options may be available; check
in the manual of your mail user agent package for details.  You may
also define another mail user agent using
@code{define-mail-user-agent}.

  If you select a different mail-composition method, the information
in this chapter about the mail buffer and Message mode does not apply;
the other methods use a different format of text in a different
buffer, and their commands are different as well.

@vindex read-mail-command
  Similarly, to specify your preferred method for reading mail,
customize the variable @code{read-mail-command}.  The default is
@code{rmail} (@pxref{Rmail}).