Custom Validation Rules Pattern

I have not worked a lot with the Business Rule Engine (BRE) from BizTalk, but today I had to show a customer how it worked. I demoed a BRE solution, where a rule was applied to message taken by an sample orchestration (see picture below, click to enlarge).Then customer asked if BRE could be used to validate messages received from a third party by applying rules. I said that it was possible, but