As you compose emails in Outlook, it keeps track of the email addresses you type in and then later suggests those addresses as autocomplete entries.  If you find that an autocomplete entry is broken in Microsoft Outlook for any reason, you can delete the entry as follows:

Compose a new email

In the To: field, being to type in the offending address until the autocomplete box appears with the address in it.

Press the down arrow key on your keyboard to select the problem address.

Hit the Delete key on your keyboard.

The problem entry should now be removed from your autocomplete cache!