Abstract: In a network communication system passing messages between gateways via a message handling system the gateways are interfaced specifically to their respective network access units and are interfaced generically to the message handling system using routines common to all gateways. Messages are sent in protocol data units including recipient addresses which do not identify recipient gateways as such; the gateways are used transparently. The data format is CCITT 1988 X400 standard with automatic conversion to and from this format at sending and receiving gateways plus automatic document conversion. Message handling involves waiting for many services and events. The invention allows calling routines to avoid pending while waiting for events and services. Service routines, including event watching and timer routines, schedule notifications on to queues and the main processing task runs notifications off the queues by calling a run routine.