how to validate if an email actually exists from the DNS

Question

i am creating a web service to validate if an email inputted by a user exists or not. here is the web method.The problem is the method always returns true when the email is wrong or right.

[WebMethod(Description = "This method is used to validate email")]
public bool ValidateEmail(string email)
{
    bool isValid = false;

    try
    {
        string[] host = (email.Split('@'));
        string hostname = host[1];

        IPHostEntry IPhost = Dns.GetHostByName(hostname);

        IPEndPoint endPt = new IPEndPoint(IPhost.AddressList[0], 25);
        Socket soc = new Socket(endPt.AddressFamily, SocketType.Stream, ProtocolType.Tcp);

        soc.Connect(endPt); //open connection to host
        soc.Close();
        isValid = true;

    }
    catch (Exception ex)
    {
        //ex.Message.ToString();
        isValid = false;
    }

    return isValid = true;

 }