Hi Manu Kumar,
You can fix authorization issue by changing "Ouput Mode" for message type ORDERSP to "Collect IDocs" and schedule program RSEOUT00 as periodic background job with authorized user in step(system user who got authorization for message type ORDRSP ).
>>Strange thing is when same users can IDOC type ORDERSP can be sucsessfully sent to other SAP system through PI.
When IDOC/Message goes out to PI it uses user maintained in RFC destination unless principle propegation/or Trusted connection with current user used
Regards,
Jag