Dovecot - Dovecot is an open source IMAP and POP3 email server for Linux/UNIX-like systems, written with...
DBMail - DBMail is a fast, scalable IMAP4 and POP3 server that stores all email and users in a Postgresql...
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Courier IMAP - The Courier IMAP server is a fast, scalable, enterprise IMAP server that uses Maildirs.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.