Old names in Mail, coming from Previous Recipients file
In the Window Menu in Mail, about halfway down, select "Previous Recipients".
This opens a window where mail stores email addresses on any address you ever sent an email to.
The small card on the left means the recipient was in your address book when you sent the email.
You can sort the file by clicking any of the headers. If you click on the date column you'll get all your records from oldest to newest. You can delete any one or group that are no longer used.
In addition, if you find an address that you thought you'd lost, you can add it to the address book right from this screen.