hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Sendmail - Sendmail is a general purpose internetwork email routing facility that supports many kinds of...
PowerMTA - PowerMTA is an industrial-strength software for high-volume email delivery.
Exim - Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.
mailcow - An open source mailserver suite.