Start composing a mail message to send.
This uses the user's chosen mail composition package
as selected with the variable `mail-user-agent'.
The optional arguments TO and SUBJECT specify recipients
and the initial Subject field, respectively.
OTHER-HEADERS is an alist specifying additional
header fields. Elements look like (HEADER . VALUE) where both
HEADER and VALUE are strings.
CONTINUE, if non-nil, says to continue editing a message already
being composed. Interactively, CONTINUE is the prefix argument.
SWITCH-FUNCTION, if non-nil, is a function to use to
switch to and display the buffer used for mail composition.
YANK-ACTION, if non-nil, is an action to perform, if and when necessary,
to insert the raw text of the message being replied to.
It has the form (FUNCTION . ARGS). The user agent will apply
FUNCTION to ARGS, to insert the raw text of the original message.
(The user agent will also run `mail-citation-hook', *after* the
original text has been inserted in this way.)
SEND-ACTIONS is a list of actions to call when the message is sent.
Each action has the form (FUNCTION . ARGS).
RETURN-ACTION, if non-nil, is an action for returning to the
caller. It has the form (FUNCTION . ARGS). The function is called after the mail has been sent or put aside, and the mail buffer buried.