1. A method for propagating data by means of an intermediate service in a data processing apparatus comprising: an application for initiating outbound flows to a remote entity; the intermediate service; a delivery mechanism for processing outbound flows from the application and for notifying the intermediate service of each outbound flow; an auxiliary service, registrable with the intermediate service, for providing the data to be propagated with outbound flows; the method comprising the steps of: receiving a registration request from the auxiliary service, the request comprising a name to be associated with the auxiliary service, wherein the registration indicates that the auxiliary service requires to be called during outbound flow processing; receiving a plurality of notifications, from the delivery mechanism, each notification relating to an outbound flow; and responding to each notification by: creating a context; calling the auxiliary service to request data to be propagated; adding the name associated with the auxiliary service and any data returned from the calling step to the context; and providing the context to the delivery mechanism for inclusion with the outbound flow.