def do_USER(self, user):
        """
        Handle a USER command.

        Save the username and send a successful response prompting the client
        for the password.

        @type user: L{bytes}
        @param user: A username.
        """
        self._userIs = user
        self.successResponse(b'USER accepted, send PASS')

        Handle a USER command.

        Save the username and send a successful response prompting the client
        for the password.

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