> We need to be able to treat anonymous email differently than email from people we actually know.

The simplest solution to that would be an "only show me emails from people in my address book" filter. That would mostly echo how we treat user trust on all other platforms. Genuinely surprised this doesn't exist in most email clients (or does it and I have just overlooked it so far?)

Of course that's only a partial solution and wouldn't work for accounts where you expect unsolicited mails from people you don't know. I'd see it more as a "low-hanging fruit" solution. You could also expand the heuristic, e.g. also consider previous conversations, mailing lists, etc.

(Interestingly, the "introduce a friend" functionality would come for free: You can already send contact details as a VCard in an attachment. When receiving such a mail, some email clients will show a button to quickly add the contact to the address book.)