GNU Emacs Manual. Node: Rmail
Chapter 25: Reading Mail with Rmail
Rmail is an Emacs subsystem for reading and disposing of mail that you
receive. Rmail stores mail messages in files called Rmail files.
Reading the message in an Rmail file is done in a special major mode,
Rmail mode, which redefines most letters to run commands for managing
mail. The command
rmail-mode is used to switch into Rmail mode,
and it runs the hook
rmail-mode-hook as usual, but don't run this
command by hand; it can't do a reasonable job unless the buffer is
visiting a proper Rmail file.