Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
mailcow - An open source mailserver suite.
Cypht - All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail.
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.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.