Stop old address from auto-filling in Entourage email

Hi-

How do I delete an old email address from entourage that keeps popping up when I try to write to the person's newer address? I don't have the old one in my "contacts," it's just an address that is floating around in the system that pops up when I type their name, and if I don't check it before I send, messages go to her old address.

Thanks!