Office 365 (Outlook 2013 / 2016 for Win) - Auto-Complete Names and Addresses in the To, Cc, and Bcc Fields
The Auto-Complete List is a feature that displays suggestions for names and e-mail addresses as you begin to type them. These suggestions are possible matches from a list of names and e-mail addresses from the e-mail messages that you have sent. When you first start to use outlook, you will not have any contacts in your Auto-Complete List, but this will fill up over time.
By default, this feature is turned on in Outlook. To turn on or off the Auto-Complete List, do the following:
1. Click the File tab.
2. Click Options.
3. Click Mail.
4. Under Send messages, select or clear the Use Auto-Complete List to suggest names when typing in the To, Cc and Bcc lines check box.