A misconception that many have is that the Outlook auto-complete cache is integrated with the Outlook AddressBook which is not the case in any way.
What is the Auto-Complete cache
It’s a collection of all recipients to which an email has been sent. It gets invoked when you start entering a value in the <FullName> field when composing a new message and lists the names that match what is being entered.
To emphasize, the entries in the auto-complete cache are not those found in the AddressBook (unless you have sent a message to particular contact in your AB).
To delete an entry from the cache, just highlight an entry and click on the “X” that appears to the right of the name.
Some people like to select their recipients from the cache instead of clicking on the To/Cc/Bcc buttons which opens the AddressBook. Unfortunately there is no way in Outlook to simply add members to the auto-complete cache from the AddressBook or from the cache to a contact folder.
Fortunately, there is a free (for personal use) comprehensive tool NK2Edit from NirSoft available to completely manage the auto-complete cache allowing you to add, edit, delete, import and export entries to/from Outlook.
The program does not require any kind of install and when run automatically opens the auto-complete cache showing all entries that exist. The following screen shots show an overview of functions available
File menu options
Action menu options
Edit menu options
