Another idea is proof-of-work systems - the sender has to prove that they've performed some work. hashcash is an example, where to sender has to compute a hash

Another idea is proof-of-work systems - the sender has to prove that they've performed some work. hashcash is an example, where to sender has to compute a hash with certain properties, and use is as a 'stamp'. You'd need a whitelist system along with it, else legitimate lists with large subscription bases would get hit hard.