iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
mailcow - An open source mailserver suite.
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.
Modoboa - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.
Mail-in-a-box - Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.