Exim, which was developed by the University of Cambridge, is a free mail transfer agent.

Postfix is an implementation of a mail transfer agent (MTA) as free software.

Sendmail is an implementation of a mail transfer agent (MTA).

The courier POP3/IMAP server is an extremely stable and established server that reliably and quickly makes emails available in the Maildir.

Dovecot is a free IMAP and POP server that was developed with a special focus on security.

Amavis is an Open Source content filter for mail servers. As a stand-alone filter, Amavis provides protection against spam, malware, and viruses...