Want to elaborate on a few points I made:
1. Regarding whether POP is ubiquitous enough to be the mechanism for "pull", I think that is just details. The overall proposal is to use "pull" (in what ever form) instead of "push" email for *legitimate* bulk message delivery.
2. Regarding additional burden on *legitimate* bulk message *senders*:
a. These senders are much, much fewer than the # of receivers suffering from spam. Any incremental cost on the few is justified.
b. The senders are already quite resource savvy, else they would not be sending *legitimate* bulk (in statistical significance) messages. I doubt the incremental cost is significant to cause failure.
c. And compare this burden to the burden they have with dealing with spam, which would be eliminated by this proposal. If spam isn't eliminated, then they need not adopt the proposal. It is a self-feeding ("chicken and egg" go together) proposition.
3. Regarding additional burden on *legitimate* bulk message *receivers*, here is I think your best argument for significant pain in adoption. Of course it was also painful during the browser wars. Pain of transistion is not an unknown experience on the internet. And compare this potential pain of adoption to the very significant pain and cost of spam (which is getting only worse).
4. Regarding whether spam is significant to the economy of internet (and thus has powerful vested supporters), if it is possible that an activity that gets on average less than 0.005% response rate (or even if you assume 1%) is significant compared to ALL other internet activities combined, then that is sad statement about the internet economy. I am very confident that is not true assumption. And compare this to the massive cost of spam. There is link I could dig which estimates that the yearly cost of spam to business is $9 billion. Do I need to dig it up or can you Google for it?
Shelby Moore
http://AntiViotic.com