Ajay,
looks like your legacy receiver system does not send back a soap message but (plain) XML.
You could do the following to narrow this error down:
1. Open ICO in PI and set logging option to always log
2. If you don't see the response then in PI, take the request after the mapping step from pi, paste that into soapUI or any other soap or rest testing tool and fire that at your legacy third party service
3. I assume you will get back a plain XML
4. You would then need to switch the receiver channel to http_aae or use soap channel with option "do not use soap envelope"
Please come back if this doesn't help
Cheers
Jens