hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
Emails - Simple, easy and secure mail server with a convenient and simple interface.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
PowerMTA - PowerMTA is an industrial-strength software for high-volume email delivery.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
mailcow - An open source mailserver suite.