If you are getting your inbound messages from a folder where would the acknowledgement be sent? From the source node you can send an acknowledgement if your source is an HTTP Listener, a socket or a web service. In those cases, the acknowledgement would be sent to the sending system. As you are reading a folder to get the incoming messages you would need to use either a custom script as the source to read the folder and then send the acknowledgement or send the acknowledgement from a mapping node.