Earlier quoted context omitted.
Make it cost a few cents to send a mail with it, just like stamps of physical mail. It's mostly intended for receiving anyway.
That's the truth, really. Email should cost [$0.02] or some similar value. You'd solve spam overnight.
While extremely idealist and would never happen in practice, I think I'd rather see a provider that enforces KYC-esque requirements and is a closed iMessage-esque system. I took my phone number off my main mobile device due to spam (pretty much everyone I know has an iPhone) and have not looked back since.
If I want privacy, I'll self-host. If I don't care, I'll use the monopoly.