An application on a server submits the message to a gateway (1) (which it sees as a service centre). The gateway performs interaction with external applications on servers such as charging and then instead of storing transaction related information associated with this message, it packages the transaction information along with the message as inter-working data. The gateway then forwards the message including the inter-working data to the service center, while requesting a delivery receipt. The service centre may generate a message-id for this transaction, but no data relating to the transaction now needs to be stored by the gateway. The gateway (1) then informs the application that it has successfully stored the message for delivery. When the message reaches a complete state in the service centre the service centre (25) sends the gateway (1) a delivery receipt including the service centre generated message-id and the inter-working data. At this point the gateway no longer needs to retrieve transaction information relating to this message (based on service centre and message-id), as the data is already present in the message. On inspection of the transaction information the gateway can determine whether the original application requested the delivery receipt, and if so, forward the receipt to the application. Dependent on the resulting state of the message and any transaction information, the gateway may interact with external servers again. At this stage the gateway (1) then generates a Call Detail Record and acknowledges receipt of the delivery receipt.