This brings up a more general point about the dangers of blocking everything under the sun. When you limit yourself to just a few chokepoints, its easier for those who would stifle communications to shut things down.
This is a very dangerous path to take. Not that we shouldn't consider some sort of port restrictions to stop spam, but there are undesirable long term effects that need to be considered. Those on the dark side will be "considering" them, you may be sure, while licking their chops.
It can be built without choke points. ISPs could form trust relationships with each other and bypass the central mail relay. AOL for example could require ISPs to meet certain criteria before they are allowed direct connections. ISPs would need to contact AOL, provide valid contact into and accept some sort of AUP (I shall not spam AOL...) and then be allowed to connect from their IPs. AOL could kick that mail server off later if they determine they are spamming.
-Matt
