Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
MailStore - MailStore Home - A 100% free single-private-user desktop solution
mailcow - An open source mailserver suite.
Hornetsecurity Email Archiving - Hornetsecurity Email Archiving is one of the advanced software that offers long-term, unchangeable, and secure storage of important company information, data, and flies.
Mail-in-a-box - Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.
Intradyn Email Archiver - Orca Email Archiver provides email archiving solution for local government and business.