ASP.Net SMTP Exception: The remote name could not be resolved: smtp.gmail.com

I am using the following code to send email

When i click send button the following error occur.

    protected void Button1_Click(object sender, EventArgs e)
    {
        MailMessage mm = new MailMessage("sender@gmail.com", "receiver@gmail.com");
        mm.Subject = "Hello";
        mm.Body = "Hello";
        mm.IsBodyHtml = false;
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "smtp.gmail.com";
        smtp.EnableSsl = true;
        System.Net.NetworkCredential NetworkCred = new System.Net.NetworkCredential();
        NetworkCred.UserName = "sender@gmail.com";
        NetworkCred.Password = "xxxxxxx";
        smtp.UseDefaultCredentials = true;
        smtp.Credentials = NetworkCred;
        smtp.Port = 587;
        smtp.Send(mm);
    }