Although this sounds very simple however I had to struggle to make this working while I had to migrate an existing interface from one of our customers interface from XI/PI--->PI to XI/PI-->PO.
First of all make sure you have same names of Interfaces(Inbound Interface of the Sender and the Outbound Interface of the Receiver) and Namespace in both the system. So if your customer has defined the namespace and Interface Names then you need to follow them exactly as they are. For My example, below are the design(I have kept the screen only for those objects which may need clarifications).
Customer's XI/PI system(Sender).
Our PO system(Receiver).
Config in Customer's XI/PI system:
Please note that we need to do the Header Mapping so that the names of the Sender/Receiver Component matches the name of the Sender and Receiver Component used in PO system(Receiver).
Receiver Channel in Customers PI system:
Please note that as the receiving system is PO(No ABAP Stack) the URL pattern is as below.
http(s)://host:port/XISOAPAdapter/MessageServlet?ximessage=true
Earlier while Receiving system was PI, then below URL pattern was followed.
http(s)://host:port/sap/xi/engine?type=entry
Config in our PO system:
The rest of configuration is usual and you can follow standard design. Also please note that if you face and issue with HTTP-500 while trying to post the message from Sender however you are not able to find that in Receiving system then please check the Header Mapping in Sender system ICO and receiving systems ICO so that the System/Component/Party Names matches with the System/Component/Party Names used in receiving System as it is done in my case. If that also does not resolves the issue, double check the Namespace and Interface Name is same on the sender and the receiver system.