Users who just want to try RMAIL out to see how it works end up trapped using it because saved mail in their `mbox' file has been converted into an incompatible format (BABYL) that only RMAIL understands. RMAIL provides no obvious way to reverse this transformation. Kyle Jones has aptly named this "the great Emacs Mail Eating Monster". To convert a mail file back to standard Unix format, there are several methods:
C-x ( C-o mbox RET M-s ^From: RET M-0 C-x )(The rmail-search command ("M-s") is used instead of just "n" because it is the only command which will cause an error when it reaches the last message in the file, which is necessary to terminate the keyboard macro. This will fail if there are messages in the file that don't have a `From:' header. This assumes rmail-delete-after-output is nil.) It is wise to save a copy of the RMAIL file first, in case you make a mistake.
You may wish to disable RMAIL to avoid accidentally destroying your mbox file (I have this in my .emacs):
(put 'rmail 'disabled t) ; avoid mbox destruction