The unwanted address has been completely purged from the "Previous Recipients" list and I have run a couple of tests. It will still appear in the choice list as the first three letters are typed but only under a solid line and with the caveat: (found in mail). So I assume mail does a quick search to see if the address is elsewhere. Because it is no longer at the top of the list, it can't be accidentally used, so I'm good.
Also, as soon as the fourth letter is typed, it disappears.
