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.
sSMTP - sSMTP is a simple MTA to deliver mail from a computer to a mail server.
PowerMTA - PowerMTA is an industrial-strength software for high-volume email delivery.
Sendmail - Sendmail is a general purpose internetwork email routing facility that supports many kinds of...
mailcow - An open source mailserver suite.