If Outlook is not auto filling addresses, there is a two step process to fix this problem. Please follow the steps below:
Check if Auto-Complete option is enabled in Outlook:
1. Open Outlook
2. Select File
3. Select Options
4. Select the Mail tab
5. Under Send Messages on the right of the window select Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines (if it is already selected move to the next step)
Empty Auto-Complete List:
1. Select the Mail tab (if it is still open you can move to the next step)
2. Under Send Messages on the right side of the screen select Empty Auto-Complete List
3. Select Yes to confirm
4. Send a few test emails to ensure auto-complete is now working