Using Rmail in the simplest fashion, you have one Rmail file `~/RMAIL' in which all of your mail is saved. It is called your primary Rmail file. The command M-x rmail reads your primary Rmail file, merges new mail in from your inboxes, displays the first message you haven't read yet, and lets you begin reading.
Rmail uses narrowing to hide all but one message in the Rmail file. The message that is shown is called the current message. Rmail mode's special commands can do such things as delete the current message, copy it into another file, send a reply, or move to another message. You can also create multiple Rmail files and use Rmail to move messages between them.
Within the Rmail file, messages are normally arranged sequentially in order of receipt; you can specify other ways to sort them. Messages are assigned consecutive integers as their message numbers. The number of the current message is displayed in Rmail's mode line, followed by the total number of messages in the file. You can move to a message by specifying its message number with the j key (see section Moving Among Messages).
Following the usual conventions of Emacs, changes in an Rmail file
become permanent only when the file is saved. You can save it with
rmail-save), which also expunges deleted messages from
the file first (see section Deleting Messages). To save the file without
expunging, use C-x C-s. Rmail also saves the Rmail file after
merging new mail from an inbox file (see section Rmail Files and Inboxes).
You can exit Rmail with q (
rmail-quit); this expunges and
saves the Rmail file and then switches to another buffer. But there is
no need to `exit' formally. If you switch from Rmail to editing in
other buffers, and never happen to switch back, you have exited. (The
Rmail command b,
rmail-bury, does this for you.) Just make
sure to save the Rmail file eventually (like any other file you have
changed). C-x s is a good enough way to do this
(see section Saving Files).