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.
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.