def _flush(self):
        """
        Fire whatever Deferreds we've built up in our mailbox.
        """
        while self.pop3Server.mbox.waiting:
            d, a = self.pop3Server.mbox.waiting.pop()
            d.callback(a)
        IndexErrorCommandTests._flush(self)

        Fire whatever Deferreds we've built up in our mailbox.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_pop3.py