MailStore - MailStore Home - A 100% free single-private-user desktop solution
MailSteward - MailSteward : The Ultimate E-mail Manager for Mac OS X
Gmvault - Backup the emails in your Gmail account.
imapsync - Console-based utility for migrating IMAP mailboxes.
Loop Email - Emailing for teams: Organize your Inbox into channels 💌
Mail Archiver X - Mail Archiver X protects one of your Mac-based small business' most important assets.