Forwarded mail are created as response documents in MailIn database

I have upgraded the applications from R6/7 to R8.5.2 environment. However, we are facing a very strange problem with Mailin database.Below is the scenario for the issue:

  1. R6/R7:- a. If a new mail is sent from outlook to application mailin database, its getting created in Mailin database with out any issue.

        b. If a mail is forwarded from out look to application mailin database, its getting created in Mailin database with out any issue.
    
  2. R8:- a. If a new mail is sent from outlook to application mailin database, its getting created in Mailin database with out any issue.

b. If a mail is forwarded from out look to application mailin database, its getting created as response document with out parent document in Mailin database i.e. the document is having “$REF” field.

I am unable to find why document is creating in the Mailin database as response. I would appreciate any suggestion to overcome this issue.