Post Xml Data Back To External Resouces In B2B Solution

When an external resouce programmatically post the XML document as content
of http post request, and at server-side, I can read the XML data from http
request's body. Just like this example provided by Steven Chen:

#Happy Days Are Here Again: Posting XML to the Server
http://msdn2.microsoft.com/en-us/library/ms950790.aspx

After I process the data, I need to write a response message to the external
resource in xml data format(either succeful messge or failed message). what
people usually do, do I just write the xml message to response output
stream(let them extract from my page themselves) or
post data back to their page (which initially post/submit data to my server
program).