I create a mail class to send emails. 

public void SendEmail(string subject, string messageBody, string toAddress)
{
    MailMessage mail = new MailMessage();
    mail.To.Add(toAddress);
    //mail.To.Add("amit_jain_online@yahoo.com");
    mail.From = new MailAddress("noreply3ncra@gmail.com");
    mail.Subject = subject;
    string Body = messageBody;
    mail.Body = Body;
    mail.IsBodyHtml = true;
    SmtpClient smtp = new SmtpClient();
    smtp.UseDefaultCredentials = false;
 
    smtp.Host = "smtp.gmail.com"; //Or Your SMTP Server Address
    smtp.Credentials = new System.Net.NetworkCredential
         ("noreply3ncra@gmail.com", "********");
    //Or your Smtp Email ID and Password
    smtp.EnableSsl = true;
    smtp.Send(mail);
}

But I got this error: 

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. 

So I have to sign into Gmail and enter the captcha code and after this every thing is going to be ok. 

What should I do?