Phantom e-mail address book

I recently changed the listed ISP of one of my email addressees (was Adelphia, now Comcast), but when I send him email (to the new address) I keep getting an error message saying the address is no good, and identifying it as the one I had deleted (Adelphia). I normally use Entourage, but I get the same problem using Mac's Mail, whose address book is linked to Entourage's. Is there a hidden address book I need to unearth to finally drive a stake through the heart of the Adelphia listing?