I have built and released an open-source email server in the past for testing send rates and speeds, this project was called SimpleMTA and is available here.
Recently I have rebuilt this project for an internal project at work using the Tornado framework. Sadly this project as a whole cannot be released but a version of this code will be released in the near future.
Until that is released I have launched a new service called blackhole.io
What is blackhole.io?
blackhole.io is a completely open mail relay that forgets anything that is sent to it, meaning there is no auth requirements and no storage of email data within the service. Literally anyone can send anything to it and have it never get delivered.
You can even send commands out of order, meaning you can call the DATA command without ever using HELO, MAIL FROM or RCPT TO …