About Frequent contacts in Outlook
Outlook provides a cache of email addresses as you use them in new email messages. This cache, sometimes called a nickname cache, is intended to improve user productivity. As you type an address in an email address field, Outlook lists possible addresses matching the letters you’ve entered. Autocomplete is a productivity enhancement that shows an address as you start to type a common recipient address so you can select the address instead of typing it
Clearing the Auto-Complete Address Cache
Outlook has an auto-complete cache to help fill in recipient information when adding recipients. If you want to delete this auto-fill cache, you can delete individual items or the entire cache. You remove individual items by typing an address on the To line of an email, and when the auto-fill suggestion is displayed, press the Delete key.