MailerQ - MailerQ is a high performance Mail Transfer Agent (MTA) designed for delivering large volume email messages at very high speeds.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
turboSMTP - turboSMTP is a reliable, powerful, easy-to-use SMTP server that maximizes the deliverability of all...
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.
SMTP - SMTP provides scalable affordable email infrastructure for all your sending needs.
mailcow - An open source mailserver suite.