How to remove Auto Complete cache entries, one at a time
Outlook 2010
Open a new email.
Type the first few characters of the cache entry that you want to remove.
When the name appears in the “suggested names” list, move your mouse pointer over the name until it becomes highlighted. (But do not click the name).
When the “X” icon appears next to the highlighted name, click X to remove the name from the list.
Outlook 2007 or Outlook 2003
Open a new email.
Type the first few characters of the cache entry that you want to remove.
When the entry appears in the “suggested names” list, use the UP and DOWN arrow keys on your keyboard to select the entry, and then press DELETE on your keyboard.