http://www.brainbell.com/tutors/XML/XML_Book_B/Sending_Messages_Using_M_POST.htm
You can restrict messages coming through a firewall or a proxy server by using the MPOST method instead of POST. M-POST is a new HTTP method defined using the HTTP Extension Framework located at http://www.w3.org/Protocols/HTTP/ietf-ext-wg. This method is used when you are including mandatory information in the HTTP header, just as you used the mustUnderstand attribute in the SOAP header element.