I have a site where a minority of users are causing trouble by registering with one email address and then changing to another email address. Email Change Confirmation helps with this, but I would also like to keep a record of changed email addresses per-user, preferably in the Drupal UI.
I am thinking about making an email address field and using rules to automatically add an old email address when the user changes his or her email, but if there is a more robust solution I would love to know about it.