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.
HEY - Email at its best, new from Basecamp.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
mailcow - An open source mailserver suite.
InstAddr - Disposable email address application.