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.
Kolab Community - Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
InstAddr - Disposable email address application.