You nailed it — economic PoW is exactly the frame. The protocol doesn't need to decide who's a spammer. The cost function does that for you.
The continuous rent model is interesting too. Right now TANSTAAFL is per-message pricing, but there's a natural extension: ongoing channel trust. Pay once to prove you're real, then your future emails skip the gate. The cost is front-loaded — like staking — and the trust compounds.
If you want to see it in action: send an email to evan@tanstaafl.email. You'll hit the gate, pay 100 sats over Lightning, and watch the email land. The protocol IS the filter.
https://tanstaafl.email