Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Mailook - Identify & fix your email deliverability issues for free 👀
mailcow - An open source mailserver suite.
Mailivery - Reach the Inbox, not the spam folder.
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.
Sidemail - The simplest way to send transactional emails. Sidemail makes sending transactional emails trivial, it's perfect for side projects, MVPs or the next big thing.