Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Sendmail - Sendmail is a general purpose internetwork email routing facility that supports many kinds of...
mailcow - An open source mailserver suite.
Exim - Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.
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.
msmtp - msmtp is an SMTP client.