We could add a friend-of-friend for 1 degree of separation (Alice gets an email from Bob, she doesn't know Bob, so it would go into her Requests folder. But Charlie knows Bob and has said 'yes this guy is legit' and Alice has configured her email to trust Charlie's email recommendations up to 2 degrees of separation, so it goes into the inbox with an explanation that it was allowed by Charlie's trust of Bob).

And poof, we've poorly remembered Web of Trust from 20 years ago!

Overall I do love the idea of HTMP and if we could get it to easily fall back to standard old bad email, I think it could start as a niche and then upgrade towards HTMP over time as more clients adopted support for it.