3.10.5.1 Server Behavior

If the server forwards any message, either a request or a response, to the client that was originally received from a SIP element located outside of the enterprise network, it SHOULD insert an ms-edge-proxy-message-trust header field into the message. This header field provides information about source of the SIP element as determined by the server that is responsible for processing messages from SIP elements outside of the enterprise network. The syntax of the ms-edge-proxy-message-trust header field is described in section 2.2.15.