docs: def message_user(self, username, domain, subject, message):
          """Currently use send_message_chat and discard subject, because headline messages are not stored by
          mod_offline."""
          jid = '%s@%s' % (username, domain)
          if self.api_version <= (14, 7):
              # TODO: it's unclear when send_message was introduced
              command = 'send_message_chat'
              args = domain, '%s@%s' % (username, domain), message
          else:
              command = 'send_message'
              args = 'chat', domain, jid, subject, message
  
          code, out, err = self.ctl(command, *args)
          if code != 0:
              raise BackendError(code)