This issue can be caused by a few different things. To troubleshoot, try these steps:

1. Check the email message for any sender and recipient addresses in the From or To fields.
2. Confirm if the addresses are correct.
3. Check to make sure you are in the correct Outlook profile or email address.
4. Ensure no proxy server is used.
5. Verify auto-completion settings are enabled for the address fields.
6. Verify that the known domains in the Global Address List (GAL) are available.