In this chapter we have described the usual Emacs mode for editing and sending mail--Mail mode. Emacs has alternative facilities for editing and sending mail, including MH-E and Message mode, not documented in this manual. . . You can choose any of them as your preferred method. The commands C-x m, C-x 4 m and C-x 5 m use whichever agent you have specified. So do various other Emacs commands and facilities that send mail.
To specify your mail-composition method, customize the variable mail-user-agent. Currently legitimate values include sendmail-user-agent (Mail mode), mh-e-user-agent, message-user-agent and gnus-user-agent.
If you select a different mail-composition method, the information in this chapter about the *mail* buffer and Mail mode does not apply; the other methods use a different format of text in a different buffer, and their commands are different as well.