Your preference for a mail composition package.
Various Emacs Lisp packages (e.g. Reporter) require you to compose an
outgoing email message. This variable lets you specify which
mail-sending package you prefer.
Valid values include:
`message-user-agent' -- use the Message package.
See Info node `(message)'.
`sendmail-user-agent' -- use the Mail package.
See Info node `(emacs)Sending Mail'.
`mh-e-user-agent' -- use the Emacs interface to the MH mail system.
See Info node `(mh-e)'.
`gnus-user-agent' -- like `message-user-agent', but with Gnus
paraphernalia if Gnus is running, particularly
the Gcc: header for archiving.
Additional valid symbols may be available; check with the author of
your package for details. The function should return non-nil if it
succeeds.
See also `read-mail-command' concerning reading mail.