Methods and related apparatus are described of operating a transaction proxy to act as a conduit for a structured transaction, such as TCAP structured dialog between a first transaction peer, for instance an SMSC, and a second transaction peer, for instance an MSC. The method comprises the steps of: receiving a begin message; determining an address of the first transaction peer from the begin message; responding to the begin message by sending a continue message to the first transaction peer; and, if no abort message is received from the first transaction peer, forwarding the begin message to the second transaction peer. Embodiments of the invention employ the techniques for filtering mt-forwardSM messages containing spoofed addresses in GSM systems,