Hashcash, or BTC.

I always loved the hashcash concept and actually raised our original funding because of it (our Microsoft angels loved the idea of making spamming more expensive, and our Series A concept was tar-pitting to dissuade botnets). In the context of email sending services, we have a modern version of hashcash that we might at some point turn to. If someone can figure out how to tokenize sending at scale, then senders could pay recipients to open their emails by attaching a "tip" to each message.

If even a small fraction of legitimate email recipients altered their mail client settings to route "tipped" messages to their inbox, that would probably suffice to get senders to participate in the scheme. Senders are starved for high quality engagement data. Meanwhile, anything we can do to make spam less likely - on a relative scale - to reach the inbox in comparison to "legitimate" traffic, is a win.