And from a users's perspective, spam makes it through and legit messages end up in spam and the whole email system is more or less centralised due to attempts at combatting the issue. Those are all user facing issues.
Your ideas for client-side blocking in a federated system
may be okay, I don't know. I'm objecting to the "Email has dealt with the spam issue mostly" thing; it absolutely has not.
> the whole email system is more or less centralised due to attempts at combatting the issue
Do the big email providers even attempt to combat spam? I got an email from booking.com yesterday that literally contains the text "This message is an advertisement." From what I can tell, Gmail hasn't maintained their spam filters in at least a decade.
Your ideas for client-side blocking in a federated system may be okay, I don't know. I'm objecting to the "Email has dealt with the spam issue mostly" thing; it absolutely has not.