3.5.5.1 Receiving a Failure Response to SUBSCRIBE

If the client receives a failure response to the SUBSCRIBE request because an intermediate proxy did not support the BENOTIFY extension, it MAY set the BENOTIFYEnabled flag to false and resend the SUBSCRIBE request without the supported:ms-benotify and proxy-require:ms-benotify headers. In this case, the BENOTIFY extension is disabled.