Acknowledgement from XI

Hi,

I am integrating a legacy application with SAP CRM using XI. The integration can be asynchronous but client has legacy system requirement to receive a blank message as acknowledgement (Response) once XI receives the message. This shows that Legacy to XI synchronous & XI to CRM Asynchronous which means I have to use BPM.

Is there any way by which I can send acknowledgement to legacy system? There are many interfaces to be built using this solution and the acknowledgement will be different for different integrations.

Whats is the good solution for this?

thanks,

KP