When I compose a new e-mail and type in the first couple of initials of the email address I get a drop down with a list of past or recently used email address. Several of those the owners have changed their email server but keep the same name. How do I eliminate those past unusable emails from the drop down list.

You need to remove the nk2 file (which is what the autocomplete is).

Do a search for any .nk2 files and then delete all of them. This will completely erase the autocomplete.