kaled for e-mail czar!
The "problem" with this approach is that queued-to-be-retrieved e-mail messages would sit on the originating server until retrieved, thus making ISPs whose users send vast quantities of e-mail foot the bill for this temporary storage. They'd also have to decide how to time-out messages which were never retrieved. For those reasons, it may meet resistance; They'd all prefer to just "fire-and-forget." However, I think anything that pushes the costs back toward the sender like this is good.
I think it more likely that the ISPs will adopt whatever method they can charge the most for... :(