Security Exception

 when i sent mails through smtp.gmail.com  , i have the security exception.i could not send email .
my coding is ,
 
MailMessage m = new MailMessage();
        m.From = new MailAddress("address@domain.com");
        m.To.Add(new MailAddress("address@domain.com"));
        m.Body = "Hai Gmail";
        m.Subject = "Hai Gmail";
        m.IsBodyHtml = true;
        m.Priority = MailPriority.High;
        SmtpClient s = new SmtpClient();
        s.Host = "smtp.gmail.com";
        s.Credentials = new NetworkCredential("username","pass&quo ...