function checkKeepMePosted(formulier) {
  var msg = "";
  if (!hasInput(formulier.firstname)) {
      msg += "You did not provide a firstname.\n";
  }
  
  if (!hasInput(formulier.lastname)) {
      msg += "You did not provide a lastname.\n";
  }

  if (hasInput(formulier.email)) {
    if (!emailCheck(formulier.email.value)) {
      msg += "This email-adres is not valid.\n";
    }
  } else {
      msg += "You did not provide a e-mail address.\n";
  }

  if(msg) {
      alert(msg);
      return false;
  } else {
      OpenWindow('http://www.hollandopen.nl/keepmeposted.jsp?action=ok&firstname=' + formulier.firstname.value + '&lastname=' + formulier.lastname.value + '&email=' + formulier.email.value,'Contact','height=480,width=480, scrollbars=NO, menubar=0, toolbar=0, status=0, directories=0, resizable=1');
      return false;
  }
}