Outlook 2007

I’ve started having problems with my contacts in Outlook 2007. When I type in a contacts name the correct email address shows but when I send it goes to someone else in my contacts list.

Example I type in Joe and his email shows joe@123.com but when it sends it goes to Mary at mary@abc.net

When I try to delete his name and reenter the contact information the incorrect information keeps coming up. Any ideas how to solve this problem? It’s not every contact just a few this is happening to. I’m running Office 2007 on Windows 7 Pro on a local domain.
Thanks for your help.