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.
MailStore - MailStore Home - A 100% free single-private-user desktop solution
Athena Archiver - Athena Archiver Email Archiving Solutions provide enhanced disaster recovery options by enabling automatic off-site email storage and data replication to a mirror site.
MailSteward - MailSteward : The Ultimate E-mail Manager for Mac OS X
Thexyz - Secure premium email service with mobile sync apps for calendar, contacts, tasks, notes and FISMA compliant email archiving.
Mail Backup X - Your one stop mail backup and archiving tool for Mac.