Whenever you type an email address in the To/CC/BCC field of an email, Outlook helps out by showing you a list of email addresses that match what you have typed. This can be very helpful in quickly entering a recipient’s email address. But …
People’s email addresses change or you may have stopped communicating with certain people – yet their old email addresses keep showing up in the list.
Deleting Single Entries
It’s easy to delete individual items from the list. Type a few characters in the To field to display the list. Then use the arrow keys on the keyboard to move to the old email addresses and click Delete.
Deleting the Whole List
Use with Caution because there is no way of getting the list back!
You can also completely delete the list to start from a clean slate by following the steps in this link: