Outlook 2007 has an automatic completion for any email address that you try to type into the To or CC fields. This feature does not pull from the contacts or address book, because you may have tried to email somebody that wasn't in your contact list, and it will still add that to the auto complete list.
The problem comes in when you put in a bad address, and then every time you go to type in the email address, the incorrect one appears in the auto complete list.
To fix this problem, there are two ways around this. The easiest way is this:
Start typing in the address that shows up incorrectly.
Hit the Down arrow key to select the auto completed address.
Hit the Delete key to delete the auto complete address
The other way is by opening the profile folder and deleting the .nk2 file.
Open up windows explorer, and paste "Outlook.nk2" into the search bar.
If you have Vista, it probably won't find it so you'll have to narrow your search.
In explorer, go to the folder Users, New, Appdata and then type Outlook.nk2 into the search box. Just rename the file and you're done!
Keywords: clear auto complete Outlook 2007