GNU Emacs Manual
Z.4 Mail Mode
The major mode used in the mail buffer is Mail mode, which is much
like Text mode except that various special commands are provided on the
C-c prefix. These commands all have to do specifically with
editing or sending the message. In addition, Mail mode defines the
character `%' as a word separator; this is helpful for using the
word commands to edit mail addresses.
Mail mode is normally used in buffers set up automatically by the
mail command and related commands. However, you can also switch
to Mail mode in a file-visiting buffer. This is a useful thing to do if
you have saved the text of a draft message in a file.