Bruce J.A. Nourish wrote:
> Suppose Alice sends an email to Bob:
> 
>  * Alice's MUA hands it off to her ISP by some transport protocol. 
>    (Could be anything, it wouldn't matter).
>  * The ISP's MDA sends a notification message to the MDA for Bob's
>    domain. (At this point you could use tarpitting/blackholing/reverse DNS 
>    or anything else you feel like.) 
>  * Bob would download all of his notification messages. Each 
>    notification message has a unique tag that allows Alice's MDA to
>    find the body of the message.
>  * If Bob is sufficiently convinced by the header infomation (Sender,
>    Subject etc.) he can use his MUA to retrieve the body. 


bjan.freeshell.org doesn't resolve, so I am posting here.

Disclaimer: anti-DJB pseudotrolls can shove it!

I think you've described http://cr.yp.to/im2000.html

I think I saw a mention of BMTP on that list too. Also Bernstein
suggests ISPs become banks, so that receivers can charge unknown
senders money. I think this is what will eventually happen (in the
bazillion years that it will take) to the Internet in order to
finally solve spam and forgery issues. The key point is with filtering
it's still more expensive for end-users and admins in time and effort
and bandwidth than for a spammer to send. They just increase the flood
as the result, and we end up paying for the increase in time and
hardware/bandwidth. Filtering is NOT a "solution."

-- 
We come to love not by finding a perfect person
but by learning to see an imperfect person perfectly.
-Sam Keen