Replies to transaction emails not getting captured as cases

Our support staff handles all order related queries so we have tried to set up a procedure so that any replies to transaction emails are attached to a case.

The process should be:

1. An email is sent to the customer with the return address of the support email.

2. The customer can then hit "Reply" and respond to the email if they have any problems or questions.

3. The email is attached to the transaction and then forwarded to the support email address that treats it as a new case.

4. All future interchanges are then happen as through the case.

The process works fine until step 3.  The email is attached to the transaction and then delivered to the support email address, but a case is never created.  Since all the email from the support address is automatically forwarded to Netsuite the email never gets seen.