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."""
  
          kwargs = {
              'body': message,
              'from': domain,
              'to': '%s@%s' % (username, domain),
          }
  
          if self.api_version <= (14, 7):
              # TODO: it's unclear when send_message was introduced
              command = 'send_message_chat'
          else:
              command = 'send_message'
              kwargs['subject'] = subject
              kwargs['type'] = 'normal'
          result = self.rpc(command, **kwargs)
  
          if result['res'] == 0:
              return
          else:
              raise BackendError(result.get('text', 'Unknown Error'))