Which Trusted Source engine are you using?
I dont know, how to check this? (McAfee Web Gateway 6.8.7 build 8846, formely Webwasher).
in most cases a reverse resolution from an IP to a hostname is not going to give you a good result. Shared hosting (50k domains on 1 ip) is a comming thing.
by default the web gateway will not a lookup of the IP for the categorization of a host.
In web gateway 7 you could do this though. you could create a rule that says 'if hostname is uncategorized, do a dns lookup and then try to categorize again". there are even options directly in the trustedsource engine. but unfortunately nothing that you could do on web gateway 6.x