hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
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.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Kolab Community - Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.
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).