Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
PowerMTA - PowerMTA is an industrial-strength software for high-volume email delivery.
mailcow - An open source mailserver suite.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic 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.
EmailSuccess - EmailSuccess is an advanced MTA built to handle the most critical reliability, performance, security, and deliverability requirements.