I understand what you mean, having had to deal with SPF nonsense for a job back in the day. The SPF nonsense is what prevents selfhosting email, as you effectively point out yourself. If there were somehow a way to use federation to tackle that, it seems like it could be kinda cool.

Decentralized email? I feel like I could get behind that

Iono… Physically breaking into the Capitol sure seems like attempting to prevent the government from doing it’s government thing, which sounds like an insurrection.