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.
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.