Thanks Sigrid for the reference to SAP note. That note specifies how to link an incoming mail to existing Interaction Record.
The issue we have is, IR number itself is not stamped on the outgoing mails. When I use the same mail forms to manually send the response, "general traking text" variable is populated with current IR number. However while send auto acknowledgement via rule policy after creating an IR, the variable is blank. Also, just to let you know, I have switched on Early Assignment number too, but still IR number is not populated.
Regards,
Sandesh