A hierarchical gateway system for use in a message delivery system is disclosed, in which each tier of gateways in the hierarchy includes means for transforming incoming messages in a manner required by downstream end-user devices. The hierarchical gateway system has the advantage of being scalable and extensible while avoiding transmission bottlenecks as the number of end-user device types, and their differing system requirements, increase. The invention is operable in the context of a notification server architecture, as well as the Web.