Sorry, the post was marked as spam…
The email used is the one of the user, the subscription is a link to the user’s data and used to contain subscription specific data (like the preferences).
You cannot pull other data from WP and there are no features like the login via mail and so on. Newsletter subscribers are actually completely separated from WordPress, except for the link used only to send emails.
Unsubscription is limited to the Newsletter database, it means that “linked” subscriber won’t be mailed.