PowerMTA - PowerMTA is an industrial-strength software for high-volume email delivery.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
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.
EmailSuccess - EmailSuccess is an advanced MTA built to handle the most critical reliability, performance, security, and deliverability requirements.
mailcow - An open source mailserver suite.