A data processing system arranged for receiving over a network, according to a data transfer protocol, data directed to any of a plurality of destination identities, the data processing system comprising: data storage (event queues 31-33) for storing data received over the network; and a first processing arrangement for performing processing in accordance with the data transfer protocol on received data in the data storage, for making the received data available to respective destination identities; and a response former arranged for: receiving a message (dashed line on FIG. 4) requesting a response indicating the availability of received data to each of a group of destination identities; and forming such a response; wherein the system is arranged to, in dependence on receiving the said message: identify received data that is directed to any destination identity that is both a member of the group and a member of a subset of the plurality of destination identities, and on which processing has not yet been performed; trigger processing by the first processing arrangement in accordance with the protocol on only the identified data; and subsequently form a response based at least partly on the result of the triggered processing, wherein the response (table 40-41) is formed so as to comprise a positive indication (41) of availability of data for a destination identity of the group if the triggered processing caused data to be made available to the respective destination identity.