我需要传入一个xml 格式的参数值

        StringBuilder postXML = new StringBuilder();//获取下发配送运单信息xml
            //postXML.Append("<?xml version=\"1.0\" encoding=\"UTF-8\"?>");
            postXML.Append("<OrderExportResult><dyek><is_success>");
            postXML.Append("T");
            postXML.Append("</is_success></dyek>");
            postXML.Append("<request><jobno>");
            postXML.Append("KENTEST5022");//
            postXML.Append("</jobno><sdname></sdname>");
            //postXML.Append("<needretbill>Y</needretbill>");//
            //postXML.Append("<retbillno> KENTEST500229</retbillno>");//
            //postXML.Append("<cod_amount>168.80</cod_amount>");

            //postXML.Append("<receiver_tel>12367878777</receiver_tel>");

            //postXML.Append("<receiver_name>王ww磊</receiver_name>");
            //postXML.Append("<receiver_address>保定南二环路1083号</receiver_address>");
            //postXML.Append("<remark></remark>");//状态产生时间

            //postXML.Append("<pick_stationname>到站入库</pick_stationname>");
            postXML.Append("</request>");
            postXML.Append("</OrderExportResult>");

为何这个参数值 post 传值 在

   HttpWebResponse myResponse;

                myResponse = (HttpWebResponse)myRequest.GetResponse();//响应失败

远程服务器返回错误: (500) 内部服务器错误。