Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
mailcow - An open source mailserver suite.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
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.
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.
Emails - Simple, easy and secure mail server with a convenient and simple interface.