Asp.net Email validation

 i have to validate an email id its domain as well as its existence in yahoo or gmail etc.
i am using the following code but its not giving the results what i need- 
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

using System.Text;
using System.IO;
using System.Net.Sockets;
using System.Net;
using System.Web.Mail;



    public partial class HR_module_mn_testing_Emailvalidation : System.Web.UI.Page
    {
        string ss = "";
        protected void Page_Load(object sender, EventArgs e)
        ...