1. A method of performing message payload processing functions in a network element on behalf of an application, the method comprising the computer-implemented steps of: receiving user-specified input at the network element; intercepting, at the network element, one or more data packets comprising network layer or transport layer headers having an address of a destination which destination differs from the network element; determining, based on the one or more data packets, whether an application layer message, collectively contained in one or more payloads of the one or more data packets, matches a message classification; in response to determining that the application layer message does not match a message classification, forwarding the data packets to the destination address without performing any user-specified actions; in response to determining that the application layer message matches a particular message classification, processing at least a portion of the application layer message by performing, relative to the portion of the message, one or more actions that are (a) specified in the user-specified input and (b) associated with the particular message classification; wherein the application layer message comprises a multi-part MIME message, and wherein processing at least a portion of the application layer message includes handling each part of the multi-part MIME message separately from each other part; wherein the message is directed to the application, and wherein the actions are performed on behalf of the application; wherein at least one of the one or more actions comprises modifying the portion of the message to produce a modified message; forwarding the modified message in new data packets to the destination; wherein the one or more data packets are destined for a first destination, and wherein the new data packets are destined for one or more destinations that include a second destination that differs from the first destination; wherein the method is performed by one or more computer devices.