Phantom e-mail address book

Is there a phantom address book in my MacBook? I recently updated the ISPs for several addresses (were Adelphia, which sold to Comcast when all the Adelphia founders went to jail). But when I send email to some former adelphia subscribers using their comcast addresses, some phantom memory in the software thinks I still said "Adelphia" and rejects the message. I normally use Entourage, but Mac's own Mail does the same thing (the address books are linked so a change in one appears in the other). What's happening? Where is that hidden address book, and how can I edit or delete those old addresses?

Did you manually edit the details shown in Address Book?

Do this by double clicking on Address Book, selecting name of the recipient and hit the little edit button and change the email address.

Of course. But I can't edit an address I can't see. The problem probably lies with the ISP and is not hiding in the Mac at all.

In Mail, have you gone to Window>Previous recipients and deleted the old email addresses from there? Maybe you are being hit with the auto-fill from there, rather than getting the address from your address book.