Way to go Hal!
Do you (or anyone else) know if it's possible to specify a fall-back routing for mail in either sendmail or another mailer? Allow mail to be delivered directly to intelligently configured servers, but fall back to ISP routing for those using brain-dead configurations like Ironmail appliances or other inaccurate IP-range blocking services.
Given that we're unlikely to be able to deploy enough cluebats to cure IP-range blocking, it seems like a logical feature.