The AddResponseHeaders callback function specifies a response header for IIS to send to the client.
BOOL WINAPI * AddResponseHeaders( struct _HTTP_FILTER_CONTEXT *pfc, LPSTR lpszHeaders, LPSTR dwReserved );
AddResponseHeaders() can be used to add headers for client before the server processes the request, but not during or after HTTP_FILTER_SEND_RESPONSE notification. If you need to add a header when the HTTP_FILTER_SEND_RESPONSE notification occurs, you may do so by using the AddHeader member of the HTTP_FILTER_SEND_RESPONSE structure.