Blackhole is an MTA (message transfer agent) that (figuratively) pipes all mail to /dev/null, built on top of asyncio and utilises async def and await statements available in Python 3.5.
While Blackhole is an MTA, none of the actions performed via SMTP or SMTPS are actually processed, and no email is delivered. You can tell Blackhole how to handle mail that it receives. It can accept all of it, bounce it all, or randomly do either of those two actions.
Think of Blackhole sort of like a honeypot in terms of how it handles mail, but it’s specifically designed with testing in mind.