Click the Delete (x) icon to remove the auto-complete entry.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: If you receive undeliverable messages when sending to a person's UI email alias (typically 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. 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.
