Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.

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.

mailcow - An open source mailserver suite.

Gmvault - Backup the emails in your Gmail account.

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.

imapsync - Console-based utility for migrating IMAP mailboxes.