WritingMail in GNU Emacs is as simple as typing ‘C-x m’.

Action behind this key binding depends on ‘mail-user-agent’ variable. It is set to ‘sendmail-user-agent’ by default which means you will be using MailMode to composer your mail.

Other candidates are:

  1. ‘mh-e-user-agent’ mostly used with MH-E client.
  2. ‘message-user-agent’ for Gnus. It uses MessageMode.
  3. ‘gnus-user-agent’ also for Gnus

Rmail and VM users are likely to use the default value and thus they use MailMode.

These are the default clients shipped in the GNU Emacs distribution. There are many other MUA not part of GNU Emacs (WanderLust,Mew, etc.).

Here is a short list of how you would configure GNU Emacs

 (setq mail-user-agent 'sendmail-user-agent)
 (setq mail-user-agent 'gnus-user-agent)
 (setq mail-user-agent 'mh-e-user-agent)
 (setq mail-user-agent 'wl-user-agent)

To change the package used for composition, just add one of the above line to you InitFile.

See also MailclientConfiguration (will be soon deleted), GettingMail.

A comparison grid may be available at ComparisonOfMuas.