This is a port of VM, a mail program running in Emacs or XEmacs.

VM (View Mail) is an Emacs subsystem that allows UNIX mail to be read
and disposed of within Emacs. Commands exist to do the normal things
expected of a mail user agent, such as generating replies, saving
messages to folders, deleting messages and so on. There are other more
advanced commands that do tasks like bursting and creating digests,
message forwarding, and organizing message presentation according to
various criteria.

It was written as an alternative to the Emacs RMAIL mail reader.
VM is highly configurable and easy to use. It supports POP and IMAP
for mail retrieval, understands MIME, and reads both the standard
UNIX mailbox format and the BABYL format used by the RMAIL mailer.

Some other features:
 - thread viewing
 - virtual folders
 - gpg integration

WWW: http://www.nongnu.org/viewmail/