Recieving an answer from another Service Desk creates a new ticket

Recieving an answer from another Service Desk creates a new ticket

Hello,
As I mentioned in the title, we have Service Desk plus and another client has implemented it too.
The issue we are having is that when they answer though email to the ticket created by us, it ends up creating a new ticket. Not just that but when the status is changed or when a user asks for information. What can be done from both parties to resolve this issue?
Thanks, regards

                  New to ADSelfService Plus?