When you add recipients to an Outlook email message, you'll notice Outlook provides suggestions as you type. These suggestions are stored in the Auto-Complete List. You can refresh this list by either clearing a single saved contact or clearing the entire list. Then, the list will rebuild, adding contacts to the Auto-Complete List each time you send an email. If preferred, you can turn this feature off.
For instructions on how to remove a single saved contact, clear the entire list or disable the auto-complete feature, follow this Microsoft Guide.