One feature that Outlook offers is remembering email addresses you use and then auto-filling them when you start typing email addresses in new emails (this is very similar to Internet browsers' auto-fill feature).
A downside with this is the possibility of an email address other than the one you want being auto-filled, and even being used without your noticing. If, though, you don't want the email address in question to appear as an option at all (e.g., it is obsolete, incorrect, or no longer needed), you can delete it from the list.
- In Outlook, start a new email.
- In the To field, start typing the email address you want to delete.
- With the desired email address selected, click the X on the right to delete it from the list of stored email addresses.