Sending Email Error: The remote name could not be resolved: mail.gmail.com

User Email confeirmation on registration page showing error The remote name could not be resolved

Server Error in '/' Application.
The remote name could not be resolved: 'mail.gmail.com'
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Net.WebException: The remote name could not be resolved: 'mail.gmail.com'

Source Error: 


Line 242:            smtp.Credentials = NetworkCred;
Line 243:            smtp.Port = 465;
Line 244:            smtp.Send(mm);
Line 245:        }
Line 246:    }

 private void SendActivationEmail(int userId)
    {
        string constr = ConfigurationManager.ConnectionStrings["DB"].ConnectionString;
        string activationCode = Guid.NewGuid().ToString();
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand("INSERT INTO UserActivation VALUES(@UserId, @ActivationCode )"))
            {
                using (SqlDataAdapter sda = new SqlDataAdapter())
                {
                    cmd.CommandType = CommandType.Text;
                    cmd.Parameters.AddWithValue("@UserId", userId);
                    cmd.Parameters.AddWithValue("@ActivationCode", activationCode);
                    cmd.Connection = con;
                    con.Open();
                    //
                    cmd.ExecuteNonQuery();
                    con.Close();
                }
            }
        }
        using (MailMessage mm = new MailMessage("hellokail@gmail.com", txtEmail.Text))
        {
            mm.Subject = "Account Activation";
            string body = "Hello " + txtUserName.Text.Trim() + ",";
            body += "<br /><br />Please click the following link to activate your account";
            body += "<br /><a href = '" + Request.Url.AbsoluteUri.Replace("Default.aspx", "UserActivation.aspx?ActivationCode=" + activationCode) + "'>Click here to activate your account.</a>";
            body += "<br /><br />Thanks";
            mm.Body = body;
            mm.IsBodyHtml = true;
            SmtpClient smtp = new SmtpClient();
            smtp.Host = "mail.gmail.com";
            smtp.EnableSsl = true;
            NetworkCredential NetworkCred = new NetworkCredential("hellokail@gmail.com", "<password>");
            smtp.UseDefaultCredentials = true;
            smtp.Credentials = NetworkCred;
            smtp.Port = 465;
            smtp.Send(mm);
        }
    }
}