MDN-Gateway

This header field is present in MDN messages (MDN = Message Delivery Notification) Indicates the name of the gateway or MTA that translated a foreign (non-Internet) message disposition notification into a MDN. This field MUST appear in any MDN that was translated by a gateway from a foreign system into MDN format, and MUST NOT appear otherwise. The format is: mta_name_type; mta_name. The MTA-name-type will normally be "smtp", and the mta-name will be the Internet domain name of the gateway.

Example:

MDN-Gateway: smtp; something.somedomain.com