[bsder]

It's not trivial, but it's doable.

Excision Mail which runs on OpenBSD hits the majority of what you need technically. https://github.com/Excision-Mail/Excision-Mail

The bigger problem is finding a hosting provider that hasn't had their entire space blacklisted.

For that, you're likely going to have to pick a "responsible" provider, have a couple of rounds of back and forth with them to prove you're neither an idiot nor a spammer, and ask them to manually open the port for you. And they're going to demand something that will tie to identity.