Outlook has a list of stored email addresses that you have used in the past.  Sometimes these are corrupt because of a typo or the address may have changed.  This can cause you to receive bounce messages.  Sometimes it is necessary to delete one of these saved addresses.  Here's how you do it.

Start composing a new email in outlook.  

Start typing the recipients email address and recommendations will begin to populate from the auto-complete list.

Find the address that needs to be removed from the list and click the 'X' to delete the address from the stored auto-complete list.

The next time you email that contact you will need to enter their full email address in the from field.