Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
Mailtraq - Mailtraq is a commercial groupware and webmail server system for the Windows operating systems.
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.
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.
Kerio Connect - KerioConnect is a commercial mail and groupware server.