Remote name could not be resloved

I am Developing an apps that send email on proxy server.By using PHP Script 
its works fine on local pc.But when code runs on windows server 2008 IIS.its create exception.Remote name could not be resloved.Pls help me.Thanks in advance for your valuable time

<pre lang="c#">
 WebClient wc = new WebClient();
        var data = new System.Collections.Specialized.NameValueCollection();
        data.Add("name", username);
        data.Add("emailid", emailid);
        data.Add("dept", dept);
        data.Add("cdept", compliantdept);
        data.Add("message", message);
        //data.Add("Password", "SecretPassword");
        byte[] result = wc.UploadValues("http://www.********.com/sendingmail.php", "POST", data);