Exchange 5.5 relaying issue

We have an Exchange 5.5 Server that seems to be allowing relaying and we can’t prevent it. We have set Routing Restrictions to allow only hosts and clients that successfully authenticate but it and when we run the RSS test all tests pass except 1 that seems to use a “%” sign instead of the “@” sign in the test recipient’s email address.
We have tried everything and are in danger of being disconnected from our ISP – any help would be greatly appreciated.