alt Hacker NewsI don't think the protocol is necessarily the problem. For example we don't say the HTTP protocol is the problem when spammers abuse website comment forms or forums, we say it's the server on the other side.
I think the answer is somewhat the same as where we've gone with many HTTP servers: proof of work. Just like Captcha and more recently Cloudflare turnstile required you complete a task before you'd be able to access as website, senders should be required to complete a task before you'll accept their email.
It can even be a sliding scale: the higher you want the chances of the recipient seeing it to be, the more work you need to do.
However this also break emails considered "legitimate" by businesses, like marketing newsletters and other nonsense, which is why it'll likely never happen.
The legacy compatibility of the protocol has brought all the hacks on top of it for identity verification like SPF, DMARC, DKIM ...
Even with those, the amount of farmed accounts from a reputable platforms is still high, and it will go higher with the cheap AI targeting that will make the texts much more well crafted and spam filters much more aggressive.
My other conjecture is that the big mail providers would have enough data to catch the spammers based on a number of signals.