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 the following into the address bar: %APPDATA%\Microsoft\Outlook Delete the Outlook. NK2 file that you find in this folder.