Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
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.
Eudora Internet Mail Server - Eudora Internet Mail Server is a highly advanced internet mail system that supports the all kind of email systems and clients either it is IMAP, POP3, SMTP and much more.
Modoboa - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.