IM2000 is a pull-based mail transport proposal where the sender stores the mail and the recipient is just pinged to go collect it. (via Mark Baker)
It's particularly interesting to think about in terms of feeds and feed aggregation. Mailing lists just become feed aggregations, for example.
If you look at the proposed IM2000 architecture from Jonathan de Boyne Pollard, there's a lot you could do with the Atom format and Atom publishing protocol (APP):
"Message stores" could just be APP servers; the Message Store Originator Access Protocol would just be APP. The Message Store Recipient Access Protocol would just be Atom over HTTP. The Recipient Notification Agent Submission Protocol is just a form of trackback (which in turn could just be a specialised APP POST, in which case the Recipient Notification Agent could just be an APP server and the Recipient Notification Agent Query Protocol just APP as well).