The autocomplete list of eMail addresses is a feature which displays suggestions to you based upon whom you have had previous correspondence. In the event that there are addresses which you wish to remove because they are no longer current or accurate do the following:
Create a new eMail message.
Begin typing the address you wish to remove.
When the address is in the autocomplete list, highlight it by using the up and/or down arrows on your keyboard.
press the delete key, or if your version of office supports it, click the X next to the name.