def do_QUIT(self):
        """
        Handle a QUIT command.

        Remove any messages marked for deletion, issue a successful response,
        and drop the connection.
        """
        if self.mbox:
            self.mbox.sync()
        self.successResponse()
        self.transport.loseConnection()

        Handle a QUIT command.

        Remove any messages marked for deletion, issue a successful response,
        and drop the connection.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/pop3.py