When in Apple Mail you compose a new message most people often type the first part of a name to make a list pop-up with suggestions. It can happen that a wrong email address shows for someone even though it is not in their address book card.

The way to get rid of old and/or unused email addresses is this:

In Mail go to the Window menu > Previous recipients.


Mail will now show a list of all email addresses you have ever sent mail to, including old, incorrect and no longer in use ones. From here you can delete or add to address book.

If you email often and have been using Mail for a long time this can be quite a long list, luckily the window features a search that makes things a lot easier. The previous recipients window is also good for email address recovery in case you lost or can't remember an email address and the person is not saved in your Address Book.