Outlook AutoComplete List (Cache)

Outlook maintains the AutoComplete list. The list is used by both the automatic name-checking feature and the automatic completion feature. The AutoComplete list, also known as the nickname cache, is generated automatically when you send email messages from Outlook. The Auto-Complete List is a feature that displays suggestions for names and email addresses as you begin to type them. These suggestions are possible matches from a list of names and email addresses from the email messages that you have sent. 

Delete Individual Email Addresses (Outlook Desktop & Web)

You can delete individual email addresses from the Auto-Complete list by typing the email address into the To: field when composing an email. When the address appears, select the “X” next to it and it will be removed.

Clear Entire Address List (Outlook Desktop)

• Select “File“
• Choose “Options“
• Select “Mail” on the left pane, then scroll all the way down to the “Send messages” section
• Select the “Empty Auto-Complete List” button. If you want to turn off the Auto-Complete feature, uncheck the “Use Auto-Complete List to suggest names when typing in the To, CC, and Bcc lines“, then select “OK“.
• Select “Yes” on the dialog box that appears.