Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
mailcow - An open source mailserver suite.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
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.
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.
MailerQ - MailerQ is a high performance Mail Transfer Agent (MTA) designed for delivering large volume email messages at very high speeds.