Updating an engine using a description language

Functionality is described for sending updated engine logic to a user device. The engine logic is expressed in a description language, such as the extensible markup language (XML). The user device uses the updated engine logic to update a parse tree. The user device then uses the parse tree to process various events. By virtue of the formation of the engine logic in a description language, a network-accessible service can disseminate the engine in an efficient manner. In one illustrative application, the user device can use the parse tree to process electronic messages (e.g., Email messages) that have been received by the user device.