Have the option of showing in a tree format the tickets that are open with the customer, so that they can choose which one they want to treat. For example: You have 2 open tickets with us:

1 - Concerning the harvester

2 - Referring to a tractor

Which one do you want to talk about?

That way, the customer's response would be directed to the correct ticket, because when there are several open tickets, the response doesn't arrive in the correct order/location.