But in effect, there is a magic low-score MX record which is a network of throttling relays. Sites that know they are trusted ignore that MX record and mail you directly (and get through because they are trusted.) Sites of unknown status follow the spec and mail that MX and are throttled if not trusted. Non-trusted sites mailing directly, disregarding the spec, don't even open a socket.

This requires a large network of cooperating sites ready to run this large MX network as a central front against spam. However, as the goal of the machines is to be slow and throttling, that is not a hard engineering problem. (At least the being slow part.)