Outlook provides a feature known as "auto-complete" that allows you to type a few characters of an email address you previously sent to and it suggests and completes the rest of the address. This feature is convenient for emailing people you send to often, but can also cause issues when their UI email routing address (the actual email address behind the UI email alias) changes.
If you receive undeliverable messages when sending to a person's UI email alias (typically firstname.lastname@example.org), and the address is being "auto-completed" by Outlook, it may be the result of the person changing their routing address. Outlook caches the routing address in the auto-complete feature, so it is trying to send to the old routing address.
To correct this problem, when you start typing the first few characters of the person's name and Outlook completes the email address, instead of allowing Outlook to populate the "To" field with email address:
- Click the Delete (x) icon to remove the auto-complete entry.
- Click the To... button and re-select the person's name from the GAL (Global Address List).
After removing it once from the auto-complete cache and re-selecting it again from the GAL, you can use auto-complete in the future and your emails will send to the correct address. You may need to restart Outlook for the new name to appear.
If you have the person stored in your personal contacts and receive undeliverable messages, remove the entry from the personal contacts list and re-add the email address.