There have been various proposals for imposing a cost on sending email, to reduce the temptation to spam;
https://en.wikipedia.org/wiki/Cost-based_anti-spam_systems
I think imposing monetary cost is the wrong approach. But what about a system where verifying your email with a newsletter or mailing list issues their server a unique stamp? Which it can use as proof that you want email from them delivered.
Just thinking out loud here. Anyone have any ideas about implementing this?