def _cbMailbox(self, result, user):
        """
        Complete successful authentication.

        Save the mailbox and logout function for the authenticated user and
        send a successful response to the client.

        @type result: C{tuple}
        @param interface_avatar_logout: The first item of the tuple is a
            C{zope.interface.Interface} which is the interface
            supported by the avatar.  The second item of the tuple is a
            L{IMailbox} provider which is the mailbox for the
            authenticated user.  The third item of the tuple is a no-argument
            callable which is a function to be invoked when the session is
            terminated.

        @type user: L{bytes}
        @param user: The user being authenticated.
        """
        (interface, avatar, logout) = result
        if interface is not IMailbox:
            self.failResponse(b'Authentication failed')
            log.err(
                "_cbMailbox() called with an interface other than IMailbox"
            )
            return

        self.mbox = avatar
        self._onLogout = logout
        self.successResponse('Authentication succeeded')
        if getattr(self.factory, 'noisy', True):
            log.msg(b"Authenticated login for " + user)

        Complete successful authentication.

        Save the mailbox and logout function for the authenticated user and
        send a successful response to the client.

        @type result: C{tuple}
        @param interface_avatar_logout: The first item of the tuple is a
            C{zope.interface.Interface} which is the interface
            supported by the avatar.  The second item of the tuple is a
            L{IMailbox} provider which is the mailbox for the
            authenticated user.  The third item of the tuple is a no-argument
            callable which is a function to be invoked when the session is
            terminated.

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