The Test-Message cmdlet is a useful tool to check if Exchange transport rules and DLP policies work correctly. You can input a test message to see what happens as the Exchange transport service applies transport rules, DLP policies, and auto-label policies based on the message contents and properties. Nice as it is to have the Test-Message cmdlet, human knowledge of what transport rules should do is probably an even more important asset.
Exchange transport rules are a powerful way to apply different conditions to messages as they pass through the transport service. In this case, we add a disclaimer to calendar meeting requests with a pretty simple rule that works on the basis that it detects a special x-header in meeting requests and applies the disclaimer when the x-header exists.