Let’s be honest, PGP has major usability issues. I mean, a standard that just tells you to “figure it out” with regards to key exchange? And while I’m sure there’s plenty of people who’ve tried to make central services to handle the key exchange part, none have actually gotten any significant usage or seemingly even agreement.

I don’t think it would much reduce spam, though. If you use email in a closed environment of sorts, you already can reject email from people you don’t know. If they use trusted email providers and you require SPF and DKIM (as most modern webmail does), spoofing isn’t really a concern, at least not if you have an allowlist of senders. And if you’re not in a closed environment, presumably you’d have to share your public key very widely, making it accessible to spammers too.