Outlook 2003 contact list

I have user who was forwarded an email to respond to from her boss. They corresponded several times and the email ended up in the drop down menu for the To: box. She was doing some house cleaning and wants to remove that email address from the list along with a couple others. I’m guessing it is pretty easy to do, but I can’t for the life of me figure out how. She is running Win XP and Outlook 2003. We are running an Exchange 2003 Server (I don’t think that has anything to do about it, but not sure). That email doesn’t appear in her contacts folder.

Thanks in advance for the help.