What about speaking plain old smtp, but with transport / mailertable rules routing all mail for domain X (say AOL or MSN) to "special access" servers that have firewall ACLs allowing only connections from a restricted set of IPs?

If it's behind a firewall, then it's not on the Internet. Since NIMTP is a proposal for improving Internet email, then I think a new port number is in order. However, anyone is free to implement NIMTP peering by using a firewall to redirect new-port-number traffic to port 25 on some existing mail servers if that suits their purposes. NIMTP is lightweight technically, i.e. no new code needed, just reconfigure things a bit. The real solution resides in the agreements and the AUP that they will enforce and that has to be hashed out by the people who run mail services at the large ISPs. Not by the BGP peering folks and not by the IETF and not by the anti-spammer brigade. --Michael Dillon