forget about self hosting email... I tried it for years, and even if you get it working (needs months), it will eventually stop working again. The problem is that in order to get the big boys to accept you as an email provider, you have to jump through infinite hoops, and be treated like a criminal and/or scammer in the meantime (or at best a business that is trying to send newsletters). You will never get a human to talk to, it's just an infinite loop of automated processes.

Anyway, the problem is "trust" which boils down to IP reputation. And since we are all still on ipv4, your IP was reused. Which means you need to spend months cleaning it. And you won't have a guarantee that you won't lose this IP in the future.