Oops! There was an error in the first release (the observer class was not being instantiated).

I've just uploaded v1.1 to correct the error in class.emailFor404.php from

Code:
 function emailFor404() {
    if (SEND_404_ERRORS_EMAIL_TO_STATUS == '1' and SEND_404_ERRORS_EMAILS_TO != '') {
      $this->attach($this, array('NOTIFY_HEADER_END_PAGE_NOT_FOUND'));
    }
  }
to

Code:
 function emailFor404() {
    global $zco_notifier;
    if (SEND_404_ERRORS_EMAIL_TO_STATUS == '1' and SEND_404_ERRORS_EMAILS_TO != '') {
      $zco_notifier->attach($this, array('NOTIFY_HEADER_END_PAGE_NOT_FOUND'));
    }
  }