Also, auto-complete for addressees can be toggled on and off. For Outlook 2007, go to Tools\Options, then click the E-Mail Options button on the Preferences tab. Then, in the resultant dialog, click the Advanced E-mail Options button, and you'll find the next-to-last checkbox for 'Suggest names while completing To, Cc and Bcc fields'.
To purge the accumulated list, when Outlook is not running you can move or rename (or delete if you already have a valid backup) the .NK2 file found in your user profile's AppData\Roaming\Microsoft\Outlook folder (AppData is hidden by default).
(The current version no longer uses .NK2 files, but provides a button to clear the list adjacent to the toggle checkbox.)
https://support.office.com/en-us/art...AAA=2010,_2007