There should be some kind of reporting system where you can get entire domains / organisations blacklisted from ever sending emails to any recipient, I think a harsh stance like that is the only thing that will keep our inboxes clean
Besides incremental UX improvements over time, the email experience is not well aligned with the way things actually work. The assumption that people are good actors is a poor one and almost everyone who uses email is stuck with an unmaintainable backlog of crap as a result. Instead of further incremental improvements to the same mess, switch things up.