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

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

        @rtype: L{Deferred <defer.Deferred>} which successfully fires with
            no-argument callable which returns L{IMessage <smtp.IMessage>}
            provider.
        @return: A deferred which successfully fires with a no-argument
            callable which returns a message receiver for the destination.

        @raise SMTPBadRcpt: When messages cannot be accepted for the
            destination address.
        """
        # XXX - Yick.  This needs cleaning up.
        if self.user and self.service.queue:
            d = self.service.domains.get(user.dest.domain, None)
            if d is None:
                d = relay.DomainQueuer(self.service, True)
        else:
            d = self.service.domains[user.dest.domain]
        return defer.maybeDeferred(d.exists, user)

        Validate the address for which a message is destined.

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

        @rtype: L{Deferred <defer.Deferred>} which successfully fires with
            no-argument callable which returns L{IMessage <smtp.IMessage>}
            provider.
        @return: A deferred which successfully fires with a no-argument
            callable which returns a message receiver for the destination.

        @raise SMTPBadRcpt: When messages cannot be accepted for the
            destination address.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/protocols.py