function CheckForm(form) {
  if ((form.sender_name) && (form.sender_name.value.length == 0)) {
    alert ("Missing Name");
    form.sender_name.focus();
    return false;
  }
  if ((form.sender_mail) && (form.sender_mail.value.length == 0)) {
    alert ("Missing email address");
    form.sender_mail.focus();
    return false;
  }
  if ((form.message) && (form.message.value.length == 0)) {
    alert ("Missing message");
    form.message.focus();
    return false;
  }
  return true;
}