UPD: Never mind, that's the wrong answer as it's about HL7. Sorry about that.

There are two ways to do that:

  1. Use EnsLib.MsgRouter.VDocRoutingEngine, it has Validation setting which is a string specifying types of validation to perform. Set to 1 to block documents that don't pass default validation. Default validation checks for DocType assignment and successful BuildMapStatus. This is equivalent to 'dm': 'd' - require DocType 'm' - require successful BuildMap status. Set to 0 to skip validation. In addition configure Bad Message Handler setting to handle messages which fail validation. I'm not sure it your messages would fail BuildMap validation, but that's the easiest option to implement.
  2. Create a subclass of EnsLib.MsgRouter.RoutingEngine and implement OnValidate method to provide custom validation. You'll also need to add Validation property to settings, same as EnsLib.MsgRouter.VDocRoutingEngine  does. In OnValidate method you can perform any checks you like.