C# – Posting XML to Web service in .NET C# and recieving response

I am posting XML file to Web services using C# , but I am getting error when I am requesting the response 'Server Error – 500 – You are not allowed to access the system . Any help will be appreciated.

protected void Page_Load(object sender, EventArgs e)
    {
        WebRequest req = null;
        WebResponse rsp = null;
        try
        {
            string fileName = Server.MapPath("~\\test.xml");
            string uri = "http://212.170.239.71/appservices/http/FrontendService";
            req = WebRequest.Create(uri);
            //req.Proxy = WebProxy.GetDefaultProxy(); // Enable if using proxy
            req.Credentials = new NetworkCredential("myusername", "mypassword");
            req.Method = "POST";        // Post method
            req.ContentType = "text/xml";     // content type
            // Wrap the request stream with a text-based writer
            StreamWriter writer = new StreamWriter(req.GetRequestStream());
            // Write the XML text into the stream
            writer.WriteLine(this.GetTextFromXMLFile(fileName));
            writer.Close();
            // Send the data to the webserver
            rsp = req.GetResponse(); //I am getting error over here
            StreamReader sr = new StreamReader(rsp.GetResponseStream());
            string result = sr.ReadToEnd();
            sr.Close();
            Response.Write(result);

        }
        catch (WebException webEx)
        {
            Response.Write(webEx.Message.ToString());
            Response.Write(webEx.StackTrace.ToString());
        }
        catch (Exception ex)
        {
            Response.Write(ex.Message.ToString());
            Response.Write(ex.StackTrace.ToString());
        }
        finally
        {
            if (req != null) req.GetRequestStream().Close();
            if (rsp != null) rsp.GetResponseStream().Close();
        }
    }
        //Function to read xml data from local system
  /// <summary>
  /// Read XML data from file
  /// </summary>
  /// <param name="file"></param>
  /// <returns>returns file content in XML string format</returns>
  private string GetTextFromXMLFile(string file)
  {
   StreamReader reader = new StreamReader(file);
   string ret = reader.ReadToEnd();
   reader.Close();
   return ret;
  }