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Exactly, the key-based nature of it is what unlocks enhancements over email because there's diverse ways to achieve that... PoW, payments, WoT can all work together

Email spam mitigation is largely a reputation of the provider, there's maybe a dozen providers now that can assure high deliver-ability... rolling your own is doable but wildly impractical for individuals, and that's still subject to DNS registry etc.

A key that can use any number of relays at any time breaks down a bunch of walls, then there's meta-protocols that can go on top of that... your Lightning Pub in that example already has its own email account :)