docs: def check_undelivered(to=None):
      """Sends a notification email if any undelivered dispatches.
  
      Returns undelivered (failed) dispatches count.
  
      :param str|unicode to: Recipient address. If not set Django ADMINS setting is used.
      :rtype: int
  
      """
      failed_count = Dispatch.objects.filter(dispatch_status=Dispatch.DISPATCH_STATUS_FAILED).count()
  
      if failed_count:
          from sitemessage.shortcuts import schedule_email
          from sitemessage.messages.email import EmailTextMessage
  
          if to is None:
              admins = settings.ADMINS
  
              if admins:
                  to = list(dict(admins).values())
  
          if to:
              priority = 999
  
              register_message_types(EmailTextMessage)
  
              schedule_email(
                  'You have %s undelivered dispatch(es) at %s' % (failed_count, get_site_url()),
                  subject='[SITEMESSAGE] Undelivered dispatches',
                  to=to, priority=priority)
  
              send_scheduled_messages(priority=priority)
  
      return failed_count