def validateTo(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 L{Deferred} which becomes, or a callable which takes no
            arguments and returns an object implementing L{IMessageSMTP}. 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.
        """

        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 L{Deferred} which becomes, or a callable which takes no
            arguments and returns an object implementing L{IMessageSMTP}. 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/interfaces.py