WebService-how to send back a reject message

Hi There,

 I've published a schema webservice(Request only) and just using default XML Receive Pipeline, even though message from client are schema invalid, it get through the web service, my question is:

• if using a custom pipeline defined my the schema used by the web service, will client submit the message successfully? will client get any error letting them know the message is rejected by the web service because of wrong structure?
• if cannot, does that mean i have to publish an orchestration, put an message constructor in Exception to create a error message to send back to client?

Thanks for any advice