MailCatcher - Deploy a simply SMTP MTA gateway that accepts all mail and displays in web interface. Useful for debugging or development. (Source Code) MITRuby
OpenSMTPD - Secure SMTP server implementation from the OpenBSD project. (Source Code) ISCC/deb
OpenTrashmail - Complete trashmail solution that exposes an SMTP server and has a web interface to manage received emails. Works with multiple and wildcard domains and is fully file based (no database needed). Includes RSS feeds and JSON API. Apache-2.0Python/PHP/Docker
Postfix - Fast, easy to administer, and secure Sendmail replacement. IPL-1.0C/deb
Sendmail - Message transfer agent (MTA). SendmailC/deb