Email list owners typically do not release their lists. To retain control of the list, the list owner sends out the creative you submit. There is an indicator on the email saying the list owner is sending this email. So if a consumer unsubscribes to an email, they are unsubscribing to that list owner's email, not yours.
It's possible that a person who has unsubscribed to a rented opt-in list (an email sent on your behalf) is on your house list. You send them an email, and they say, "Wait a minute, I unsubscribed!"
Our IndyMac Bank client broke new ground in this area by convincing a list owner to provide their unsubscribes from an IndyMac Bank email campaign for inclusion in the IndyMac Bank unsubscribe list. Once IndyMac Bank signed an agreement that the names would be used for no other purposes, we were able to get most other list owners to follow suit with the help of the broker, 24/7 Real Media.
Way to go, IndyMac Bank! Customers deserve to have their wishes respected.