MailStore - MailStore Home - A 100% free single-private-user desktop solution
Gmvault - Backup the emails in your Gmail account.
Mail Archiver X - Mail Archiver X protects one of your Mac-based small business' most important assets.
MailSteward - MailSteward : The Ultimate E-mail Manager for Mac OS X
Piler - Piler is an open source email archiving solution with all the necessary features for your enterprise
FastMail - Email, calendars, and contacts to highest standards of security, privacy and reliability.