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...
MailerQ - MailerQ is a high performance Mail Transfer Agent (MTA) designed for delivering large volume email messages at very high speeds.
blat - Blat is a small, efficent SMTP command line mailer for Windows.