Override :meth:`.Concurrency.create_mailbox` to create the
        mailbox server.

def create_mailbox(self, actor, loop):
        '''Override :meth:`.Concurrency.create_mailbox` to create the
        mailbox server.
        '''
        mailbox = TcpServer(mailbox_protocol, loop=loop, name='mailbox')
        # when the mailbox stop, close the event loop too
        mailbox.event('stop').bind(lambda _, **kw: loop.stop())
        mailbox.event('start').bind(
            lambda _, **kw: loop.call_soon(self.hand_shake, actor)
        )
        return mailbox