Replying to your edit:
it doesn’t solve spammers abusing good instances
This is an instance moderation problem. If you're letting spammers in, you need to use a better application process or something similar to that. A big problem with email spam is that most email services allow anyone to sign up for free without any checks.
Ultimately defederating bad actors and defederating "good" actors who fail to moderate their own users is necessary.