Hi Li / Steffen,
As per understanding response from SAP is not required for the integration scenario , most of the cases event message response is not required , besides that SAP standard ES supports only to synchronous call .
Changes could be required in /SAPTRX/SPOT_SE_EM_SERVICES (/SAPTRX/IF_BADI_TPEN_SEND-OUTBOUND_PROCESSING) outbound processing BADI with below logic
validation flag option for outbound notification should be enabled and the same has to be validated before the confirmation proxy call.
Hope it helps to solve the issue without changing the SAP standard objects and it avoids custom asynchronous development in PI/PO ,customer can control the outbound notification by implementing the BADI .
Cheers,
Sukumar