check Email exists or not

Generally, there is no a way to check up existence of some e-mail account except sending some e-mail and getting the answer, which you, naturally, may never get, even if the account exists. This is not how mail system works.

And thing about this: it would be very insecure, would invite spam.

For more detail, please read on e-mail tracking: Email tracking - Wikipedia, the free encyclopedia[^].

You cannot rely on any of the tracking techniques, when it comes to arbitrary e-mail address. Non of them can be guaranteed to supported. I would assume it could be only useful inside a big corporation with unified technical policies.

Best wishes in New Year!

—SA

You can check the syntax of the email with such regular expression:

public static bool isEmail(string inputEmail)
{
   inputEmail  = NulltoString(inputEmail);
   string strRegex = @"^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}" +
         @"\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\" +
         @".)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$";
   Regex re = new Regex(strRegex);
   if (re.IsMatch(inputEmail))
    return (true);
   else
    return (false);
}

Or you can make SMTP check:

string[] host = (address.Split('@'));
string hostname = host[1];

IPHostEntry IPhst = Dns.Resolve(hostname);
IPEndPoint endPt = new IPEndPoint(IPhst.AddressList[0], 25);
Socket s= new Socket(endPt.AddressFamily, 
        SocketType.Stream,ProtocolType.Tcp);
s.Connect(endPt);

The following snippets are not mine, they are taken from this source:
http://stackoverflow.com/questions/3883518/can-i-check-if-an-email-address-exists-using-net[^]

Write a module and this module on form load or any other button click

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 = Encoding.ASCII.GetBytes("HELO KirtanHere" + CRLF);
netStream.Write(dataBuffer, 0, dataBuffer.Length);
ResponseString = reader.ReadLine();
dataBuffer = Encoding.ASCII.GetBytes("MAIL FROM:<youremail@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 = Encoding.ASCII.GetBytes("RCPT TO:<senderemail@domainname.com>" + CRLF);
netStream.Write(dataBuffer, 0, dataBuffer.Length);
ResponseString = reader.ReadLine();
if (GetResponseCode(ResponseString) == 550)
{
MessageBox.show("Mai Address Does not Exist !

Original Error from Smtp Server :" + ResponseString);

}
/* QUITE CONNECTION */
dataBuffer = BytesFromString("QUITE" + CRLF);
netStream.Write(dataBuffer, 0, dataBuffer.Length);
tClient.Close();

// write a sub procedure for return status of email which is valid or not
private int GetResponseCode(string ResponseString)
{
return int.Parse(ResponseString.Substring(0, 3));
}