If Outlook is not auto-filling addresses, follow these two steps to resolve the issue:
First, check if the Auto-Complete option is enabled in Outlook:
1. Open Outlook
2. Click on File
3. Click on Options
4. Click on the Mail tab
5. Under Send Messages, check Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines (if it's already checked, proceed to the next step)
Next, clear the Auto-Complete List:
1. Click on the Mail tab (if it's still open, go to the next step)
2. Under Send Messages, click Empty Auto-Complete List
3. Click Yes to confirm
4. Send a few test emails to check if auto-complete is now functioning