Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
sSMTP - sSMTP is a simple MTA to deliver mail from a computer to a mail server.
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
OpenSMTPD - OpenSMTPD is a FREE implementation of the server-side SMTP protocol as defined by RFC 5321, with...
mailcow - An open source mailserver suite.