1. A system for preventing unauthorized transmission of data over a computer network, the system comprising: a network gateway device in communication with the computer network, the network gateway device configured to receive data in transit between a source and a destination, wherein the network gateway device comprises: a classification module configured to determine whether the data in transit includes prohibited content, a context information module configured to generate destination contextual information related to the destination of the received data, wherein the destination contextual information comprises a categorization of an Internet Protocol (IP) address of the destination, and wherein the categorization of the IP address of the destination is based at least in part on website content stored at the destination, and a transmission policy module configured to determine a transmission policy based on the determination of the classification module and the destination contextual information.