Could this be addressed with cryptography, digital ids and signatures? Imagine it were possible to add a signature that proves that I own some "human" identity (like a national id), or that I possess some scarce resource (like a github account with some level of activity) and that today I sent no more that 20 emails. If I want to conceal my identity, I can use zero knowledge proofs. If you don't sign this way, or if your daily email counter exceeds 100, your mail ends up as spam.
yeah, hashcash is a very neat idea! but had problems like: how do you determine the threshold for the amount of work to be proven? there are values that makes it too expensive for a well intentioned human, and not enough for a bad intentioned spammer... moreover, it would induce economies of scale like it happens in bitcoin mining (spammers would invest in ASICs etc.) Signatures, on the other hand, allow to cheaply leverage other forms of "capital" (digital identity, github activity)
