def do_CAPA(self):
        """
        Handle a CAPA command.

        Respond with the server capabilities.
        """
        self.successResponse(b"I can do the following:")
        for cap in self.listCapabilities():
            self.sendLine(cap)
        self.sendLine(b".")

        Handle a CAPA command.

        Respond with the server capabilities.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/pop3.py