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...
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.
msmtp - msmtp is an SMTP client.
OpenSMTPD - OpenSMTPD is a FREE implementation of the server-side SMTP protocol as defined by RFC 5321, with...
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).