Mulberry is a high-performance, scalable, and graphically groovy internet
mail client. It uses the IMAP (IMAP4rev1, IMAP4, and IMAP2bis) protocol for
accessing mail messages on a server, the standard SMTP protocol for sending
messages, and does lots and lots of things with MIME parts for mixed text
and "attachments" of many different types of files and data. Support for
POP3 and Local accounts, full disconnected IMAP support, PGP/GPG,
SSL/STARTTLS, local and SIEVE support for filtering and much more!

WWW: http://www.cyrusoft.com/mulberry/