Users will inevitably try to respond to tickets that have been closed. The add-on provides two options for dealing with these messages.
You can find these options under the TICKETS->SETTINGS->Email Piping tab.
The options allow you to reject the reply and send a rejection email or to accept the reply and re-open the email.
If you choose the option to reject the message, you can configure the notice that the customer receives under the TICKETS->SETTINGS->E-MAILS tab.