It seems that Sprint has been added to the Open Relay DataBase...
> Has anyone here had this happen to their ISP? How long did it take to > get it resolved?
Every now and again, one of my ISP's mail servers will find its way onto a RBL (realtime blackhole list) such as that. Usually they get it resolved in a couple of hours. But my ISP is pretty proactive about stomping on spammers and people with open relays. Maybe if the RBL proprietors thought they were lax, it might take a bit longer to get off. (And every RBL has its own criteria, one or two have reputations of being very slow to unlist.)