User is logged on the site.

He click to subscribe to a page, and his email is stored in ametys-internal:subscriberMails

If his email changes, he will never be able to unsubscribe to this page with the old email (unless if the page is available for disconnected users, then disconnect, subscribe, set the old email, and unsubscribe).

And he will have to re-subscribe to all pages.

For logged users, the user-identity should be used to be sure to still have subscriptions.