In the system:

• People who want to use it to filter spam set up special low priority MX records.
• People who know they are on the trusted list are encouraged to, but not required to, modify their SMTP sender to ignore the special MX records (they have a magic name)
• People who are not on the trusted list, or who have a rules-conformant SMTP sender relay mail through the magic MXs. It delivers single mail but not bulk mail.
• People who try to bypass the special MX servers who are not on the trusted list are rejected immediately, perhaps with errors the first few times for politeness.
• People who are on the trusted list with a modified SMTP sender just send mail directly to the lowest priority MX that does not have the magic name. Ie. it’s just the same as before. Peer to peer SMTP, highly efficient.

If most people join the trusted list (and all list hosts need to) then mail operates just as today. No sender is forced to change a thing which is essential. For trusted senders, it’s as though the throttles are not there (just like today).