mailcow - An open source mailserver suite.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
Sendmail - Sendmail is a general purpose internetwork email routing facility that supports many kinds of...
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.
Exim - Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.