c# winform 向服务器发送XML数据问题？

我程序代码如下：
 string cid = textBox1.Text.Trim();
            string strId = textBox2.Text.Trim();
            string strPassword = textBox3.Text.Trim();
            string postData = "<?xml version='1.0' encoding='iso-8859-1'?><!DOCTYPE jds SYSTEM '/home/httpd/html/dtd/jds2.dtd'><jds><account acid='" + cid + "'loginid='" + strId + "' passwd='" + strPassword + "'><change_pwd>NEW_PASSWORD</change_pwd></account></jds>";
            //string postData = "<account acid='" + cid + "'loginid='" + strId + "' passwd='" + strPassword + "'><change_pwd>NEW_PASSWORD</change_pwd></account>";
            Encoding encoding = Encoding.UTF8;
            byte[] data = encoding.GetBytes(postData);
            System.Net.HttpWebRequest req = (System.Net.HttpWebRequest)System.Net.HttpWebRequest.Create("https://smsmgr.three.com.hk/servlet/corpsms.jdsXMLClient2");
            req.Method = "POST";
            req.ContentType = "application/x-www-form-urlencoded";
            req.ContentType = "text/XML";//SOAP
            req.ContentLength = data.Length;
            System.IO.Stream newStream = req.GetRequestStream();
            //   发送数据   
            newStream.Write(data, 0, data.Length);
            System.Net.HttpWebResponse res = (System.Net.HttpWebResponse)req.GetResponse();
            MessageBox.Show(res.StatusDescription.ToString());
            newStream.Close();            //// 获取响应
            HttpWebResponse myResponse = (HttpWebResponse)req.GetResponse();
            StreamReader reader = new StreamReader(myResponse.GetResponseStream(), Encoding.UTF8);
            string content = reader.ReadToEnd();
            //Response.Write(content); 
            textBox4.Text = content;要求发送的xml格式如下：
<!ELEMENT change_pwd (#PCDATA)>Example:
<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE jds SYSTEM "/home/httpd/html/dtd/jds2.dtd">
<jds>
    <account acid="account id" loginid="login id" passwd="password">
        <change_pwd>NEW_PASSWORD</change_pwd>
    </account>
</jds>
现在的问题是 得到的服务器返回来的数据不正确！ 告诉解答