If you want to host your own email server, then do that. As someone earlier in the thread said, email was the first federated messaging system. That was the entire point of SMTP. Anyone could host one and trust it would work.

I don’t know if it’s what you’re looking for, as it’s not ActivityPub based, but Postmoogle is a Matrix bridge email server, where you can send emails to it and they’ll be converted to Matrix messages. There might be a similar thing for ActivityPub?