1. In Outlook, click File > Options.
2. Select Mail.
3. Scroll roughly halfway down until you see Send messages. Ensure the Use AutoComplete List to suggest names when typing in the To, Cc, and Bcc lines box is checked.
4. If it's already turned on, your first troubleshooting step should be to clear out the Auto-Complete list. Click the Empty AutoComplete List button.
5. You'll see a confirmation window about clearing the AutoComplete list, Click Yes.
6. Try sending a few test emails to the same email address.