auto-Please send again

Even good spam-stopping ideas (I don't think clever filters are good) often require changes for at least two of: receivers, senders, ISPs. Here is a receiver-mostly change:

A program on the receiver's end checks the mail periodically and lets mail from people in the approved list through. Each other mail is returned (to the "From:" or "Reply-to:" address) with a note: "Sorry, by some glitch the mail filter didn't recognize you, <sender's email address goes here>. Don't panic! Just reply to this message with all of the following text included:" and the next line looks like some computer-generated diagnostic code, with a 128-bit random number added. This is followed by the sender's original text.

Most spammers don't give valid return addresses, and certainly most aren't going to spend the human time to respond, even if they get the bounce.

But if a human sends the message again, the receiver's filter matches the From address (this handles "postmaster" bounces) and 128-bit code and lets it through. The recipient can see that there was a hickup, and add the sender to his friend or enemy list.

Obviously spammers *could* write programs to handle this, but only by putting working return addresses in their spam. In the mean time it would give people a respite and incentive to think about better solutions like hashcash.

Also, anything that became popular and worked for a while would put some spammers out of business.

Details:

The recipient would configure his mail program to talk to the filter program on his own machine, and configure the filter to talk to the ISP.

This could be tweaked to send only one bounce (per month) to a given address, to limit annoyance to mailing lists you forgot to put in your friend list.