how to check the existence of the email

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Net.Mail;
using System.IO;
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Configuration;
using System.Net.Sockets;

namespace Email_sender
{
    public partial class Form1 : Form
    {

        public Form1()
        {
            InitializeComponent();
        }
        private void button1_Click(object sender, EventArgs e)
        {

TcpClient tClient = new TcpClient("gmail-smtp-in.l.google.com", 25);
            string CRLF = "\r\n";
         byte[] dataBuffer;
         string ResponseString;
        NetworkStream netStream = tClient.GetStream();
         StreamReader reader = new StreamReader(netStream);
        ResponseString = reader.ReadLine();
        /* Perform HELO to SMTP Server and get Response */
         dataBuffer = BytesFromString("HELO KirtanHere" + CRLF);
         netStream.Write(dataBuffer, 0, dataBuffer.Length);
         ResponseString = reader.ReadLine();
         dataBuffer = BytesFromString("MAIL FROM:<haseebawan0@gmail.com>" + CRLF);
         netStream.Write(dataBuffer, 0, dataBuffer.Length);
         ResponseString = reader.ReadLine();
        /* Read Response of the RCPT TO Message to know from google if it exist or not */
         dataBuffer = BytesFromString("RCPT TO:<"+txtemail1.Text.Trim()+">"+CRLF);
         netStream.Write(dataBuffer, 0, dataBuffer.Length);
        ResponseString = reader.ReadLine();
        if (GetResponseCode(ResponseString) == 550)
        {
            MessageBox.Show("ERRor!ITS not exist");
        }
        else
        {
            MessageBox.Show("Email Exist");

        }
        /* QUITE CONNECTION */
         dataBuffer = BytesFromString("QUITE" + CRLF);
         netStream.Write(dataBuffer, 0, dataBuffer.Length);
         tClient.Close();
     }
     private byte[] BytesFromString(string str)
     {
         return Encoding.ASCII.GetBytes(str);
     }
    private int GetResponseCode(string ResponseString)
     {
         return int.Parse(ResponseString.Substring(0, 3));
     }

        }