AOL rejecting mail from IP's w/o reverse DNS ?

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