def handle_default(self, line):
        """
        Handle responses from the server for which no other handler exists.

        @type line: L{bytes}
        @param line: A received line.
        """
        if line[:-4] == b'-ERR':
            self.mode = NONE

        Handle responses from the server for which no other handler exists.

        @type line: L{bytes}
        @param line: A received line.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/pop3.py