- (a) It provides an easy way for a spammer to tell if a piece of mail passes through a SpamAssassin filter, by monitoring hits on their NS.
You could give the URIBL rules first shot at the raw name, then invoke the normal lookup only if those don't get a hit.
- (b) it's pretty common in some groups to mail around unregistered domains/unresolvable hostnames/XML DTD locations/etc.
Yep, that's a hard one.
