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 can be convenient for emailing people you send to often, but can also cause issues when that person's UI email routing address (the actual email address behind the UI email alias) changes.
If you receive undeliverable messages when trying to send 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 actually 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 should be sent to the correct address. You may need to exit Outlook and restart it for the new name to show up in the list.
In addition to auto complete, if you have the person stored in your personal contacts list and receive undeliverable messages, you will want to remove the entry from the personal contacts list and re-add the email address.