If one of my contacts changes their email address I edit it and the new address shows on the contacts list but when I compose a new email I get the whole list of their previous email addresses which I don't want.
If you are on the BTMail system, log onto your account and click on "contacts". Once the list of contacts opens, go to "auto complete". Look for the relevant contact and hover over it and you will see the delete trash bin. Click on it to delete the "old" email address.
When you now compose an email this should stop the previous email address showing.