OTOH, if Earthlink (or whomever - Earthlink is just an example, here) points its customers towards something like mail.earthlink.net for SMTP relay, see below....
They run their own SMTP relays... some sort of round robin setup, servers named after different countries of the world. So, the ball is in their court, so to speak.
Take a look at somebody like Xcom (hi, marty!) - www.xcom.net. I'm not affiliated with them in any way, but it looks like what they do may be useful. A Layer 2 approach means that you can assign only _your own_ IPs to dialin customers, which cuts out the aforementioned Radius cross-reference.
That's certainly an idea worth considering, if you are not distributed accross a gazillion Ascend MAX TNT units (for UU anyways, duno what PSI uses.. anyone?)... But I digress... -=Adam