def validateTo(self, user):
        """
        Validate the address for which the message is destined.

        @type user: L{User}
        @param user: The address to validate.

        @rtype: no-argument callable
        @return: A C{Deferred} which becomes, or a callable which
        takes no arguments and returns an object implementing C{IMessage}.
        This will be called and the returned object used to deliver the
        message when it arrives.

        @raise SMTPBadRcpt: Raised if messages to the address are
        not to be accepted.
        """
        if self.delivery is not None:
            return self.delivery.validateTo(user)
        raise SMTPBadRcpt(user)

        Validate the address for which the message is destined.

        @type user: L{User}
        @param user: The address to validate.

        @rtype: no-argument callable
        @return: A C{Deferred} which becomes, or a callable which
        takes no arguments and returns an object implementing C{IMessage}.
        This will be called and the returned object used to deliver the
        message when it arrives.

        @raise SMTPBadRcpt: Raised if messages to the address are
        not to be accepted.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/smtp.py