Embodiments of the invention include techniques for processing messages transmitted between computer networks. In some embodiments, messages such as requests and responses for various types of web services, applications, and other web content may be transmitted between multiple computer networks. One or more intermediary devices or applications, such as a proxy server implemented within a physical or logical subnetwork, may receive, process, and transmit the messages between the communication endpoints. In some embodiments, a proxy server may be configured to operate within a subnetwork of an internal computer network, exposing various web applications and/or services of the internal computer network to external computer networks. Such a proxy server may select specific policies for processing messages based on various message characteristics and the current point in a predetermined processing flow for the message. After selecting the specific policies to be applied to the message, the proxy server may process the message in accordance with the policies and forward the message to its intended destination.