Mail Backup X - Your one stop mail backup and archiving tool for Mac.
imapsync - Console-based utility for migrating IMAP mailboxes.
Gmvault - Backup the emails in your Gmail account.
Piler - Piler is an open source email archiving solution with all the necessary features for your enterprise
Cryoserver - Cryoserver is an all-in-one email archiving solution that empowers you to preserve your email in a tamper-evident archive, making you transform your data into a useful archive for everyday use.
MailSteward - MailSteward : The Ultimate E-mail Manager for Mac OS X