Yeah, this is essentially a form of greylisting. The difference is (as I understand it, this is fairly outside my domain), with the current setup, MTAs can acce

Yeah, this is essentially a form of greylisting. The difference is (as I understand it, this is fairly outside my domain), with the current setup, MTAs can accept an email, and it ends up getting blackhole'd or spam-folder'd anyways. My hypothetical scheme would put more onus on the first "boundary node" to report on errors/compliance. Basically the MTA tells the client what hoops to jump through, and the client gets some indication what will happen once those conditions are met. That could be an exchange like: "Sign this nonce, and your message will be vetted", or "this is very suss, you have to do X difficulty hashes to have any chance of delivery, and regardless it'll be flagged as potential spam". Or perhaps just a guarantee on how an action would affects the message's "spam score". This could be used alongside nested packets/envelopes and various headers/trust levels in a network of trust to give a message some overall trust level.