ATT's business networks DNS department has decided to block DNS requests for the SpamCop blocking list, as well as other popular DNS-based lists, because of the extra load on their servers.
They do however provide a workaround for their customers. If you are an ATT business customer, you can contact ATT for information on secondary DNS servers which don't prevent the use of DNS-based lists.
ATT's DNS page or call 888-613-6330 (option 3,1) for more details. You will be asked to prove you are an ATT business customer.