Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
sSMTP - sSMTP is a simple MTA to deliver mail from a computer to a mail server.
mailcow - An open source mailserver suite.
Sendmail - Sendmail is a general purpose internetwork email routing facility that supports many kinds of...
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.
blat - Blat is a small, efficent SMTP command line mailer for Windows.