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.