function checkEmail(s)
{
  len = s.length;
  atsign = s.indexOf("@");
  lastdot = s.lastIndexOf(".");
  if (len > 5 && atsign > 0 && atsign < len - 4 && lastdot > atsign) {
    return true;
  }
  else {
    alert("You have provided an invalid e-mail address.");
    return false;
  }
}