Click Options from the menu list on the left.
In the Outlook Options dialog box, click Mail in the left menu list.
To disable the Auto-Complete feature, scroll down to the Send messages section on the right> uncheck the Use Auto-Complete List check box to suggest names when typing in the To, Cc, and Bcc lines.
Outlook keeps track of the email addresses used in the Auto-Complete List list. You can delete this list by clicking the Empty Auto-Complete List button.
The confirmation dialog will be displayed to make sure you want to clear the Auto-Complete List. Click Yes if you want to delete the list.
You can activate the Auto-Complete feature at any time by selecting the Use Auto-Complete List check box to suggest names when typing in the To, Cc, and Bcc lines.