Removing Auto-Complete Entries in Outlook

You can remove one or more recipients that show up as suggestions in the Auto-Complete List.

1. Enter one or more characters in the To, Cc, or Bcc boxes. Then in the Auto-Complete List that appears, select the unwanted suggested recipient by using the arrow keys or by hovering your mouse over the name you want to remove.

   

2. Choose  or press Delete.

From: https://support.office.com/en-gb/article/Manage-suggested-recipients-in-the-To-Cc-and-Bcc-boxes-with-Auto-Complete-dbe46e31-c098-4881-8cf7-66b037bce23e