Account Handed Over AFTER Cancellation Request

In August 2014 we sent in a cancellation letter to a man by the name of David. David sent us an e-mail back acknowledging receipt of the cancellation. Now OVER a year later, we receive a phone call from an attorney stating that the account has been handed over. We now owe them over R7000. The original e-mail that was sent to David was sent from our gmail account. Now ADT expects us to find this e-mail and send it to them??? The e-mail address doesn't even exist anymore let alone that e-mail! Now we must get ****ed over because David didn't do his job??? If this doesn't get resolved, I am going to the NCC.