Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
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.
mailcow - An open source mailserver suite.
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.
Roundcube - Web-based IMAP email client
MailerQ - MailerQ is a high performance Mail Transfer Agent (MTA) designed for delivering large volume email messages at very high speeds.