email existance check code it showing error?

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 Congnition Infosystem Here" + CRLF);

netStream.Write(dataBuffer, 0, dataBuffer.Length);

ResponseString = reader.ReadLine();

dataBuffer = BytesFromString("MAIL FROM:" + 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:<" + email_id.Text.Trim() + ">" + CRLF);

netStream.Write(dataBuffer, 0, dataBuffer.Length);

ResponseString = reader.ReadLine();

if (GetResponseCode(ResponseString) == 550)

{

l3.Text = "Mail Address " + email_id.Text + " Does not Exist !

";

l4.Text = "Original Error from Smtp Server :" + ResponseString;

}

///* QUITE CONNECTION */

else if (GetResponseCode(ResponseString) == 250)

{

l3.Text = "Email-id Exist
";

l4.Text = "MX record Found " + ResponseString;

}

else

{

dataBuffer = BytesFromString("QUITE" + CRLF);

netStream.Write(dataBuffer, 0, dataBuffer.Length);

tClient.Close();

l3.Text = "Email-id Exist
";

l4.Text = "MX record Found " + ResponseString;

//ScriptManager.RegisterStartupScript(Page,typeof(string),"alert","alert('Email-Id existed')",true);

}

}

 

 

"in first line Tcpclient it showing error like 'The requested address is not valid in its context 74.125.130.27:25'     Please solve how to solve this error?"