There was a similar proposal (cannot find RFC) to add it to Email protocols. Would cut spam immediately. It was even more advanced -- Recepient could choose ho

There was a similar proposal (cannot find RFC) to add it to Email protocols. Would cut spam immediately. It was even more advanced -- Recepient could choose how much sender should pay, and sender could agree to that (or do it non-interactively, i.e. send an email message with "Max-To-Pay" header, same as you can do it currently with international SMS). Unfortunately, the proposal never got any adoption, because us, email users and providers.