OpenBSD Email Service
A Python Mail Server
A reimplementation of the Python stdlib smtpd.py based on asyncio.
A Lightweight High Performance ESMTP email server
SMTP server (library) for receiving emails, written in pure PHP.
Blackhole is an MTA written on top of asyncio, utilising async and await...
SQLite-backed mail processing suite
mxcheck is an info and security scanner for e-mail servers.