I would say white-list instead of black-list if possible.
Beside the point, have you considered the reasons why you might not want to run your own email server in todays age? It’s a fun experience for sure, but if you want it for serious use it’s not for the faint of heart (unfortunately).
Edit: also lol to your friend unknowingly conducting a spam campaign from your server
Oof yeah. You’re well into admin territory here.
I mean I’m just some layman on the internet, but I would look at tying in some authentication layer to get your 2FA, although it would inconvenience your users users.
Do your users use this service for srs business?
I don’t know if I have anything else to add to this discussion. It’s gotten more complex than what “just an email server” can provide imo