Turn on or off Auto-Complete List name suggestions
The Auto-Complete List is a feature which displays suggestions for names and e-mail addresses as you begin to type them. These suggestions are possible matches from a list of names and e-mail addresses from the e-mail messages that you have sent.
By default, this feature is turned on in Outlook. To turn on or off Auto-Complete List name suggestions, do the following:
Click the File tab.
Under Send messages, select or clear the Use Auto-Complete List to suggest names when typing in the To, Cc and Bcc lines check box.