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.