This header identifies the software responsible for entering this message into the network. It has the same format as Relay-Version. It will normally identify the same site as the Message-ID, unless the posting site is serving as a gateway for a message that already contains a message ID generated by mail. (While it is permissible for a gateway to use an externally generated message ID, the message ID should be checked to ensure it conforms to this standard and to RFC 822.