why don't we consider upgrading the outdated SMTP protocol? instead receiving full message body, the receiving server should send a flag indicating that it will only accept message headers. the sending server should comply to this flag and store the full message locally, waiting for message acnowledgement.
when the user check his mailbox, he then presented with list of un-white-listed senders that trying to reach him. he has the option to accept, reject, or white-list the new senders. his isp server will then signal the sender servers to proceed sending the full messages. meanwhile it (isp server) will maintain a database of white-listed user/server pair that should be allowed to send message directly in the future.
- evolutionary, only minimal changes required on the servers and clients - on clients only simple message from their isps with clickable links much like yahoogroups
- the system will punish the spammer's isp by clogging their system with rejected messages
- there's no question for spammer with stand-alone spammer robots without SMTP server...they would be cold feeted..