Support → Email Notifications → Email Piping → Closed Ticket Warning
If you disabled the option “Allow Reply to Closed Ticket” for users, they would receive this notification if they reply to a closed ticket via email piping. You can configure the notification for the warning at Support > Email Notifications > Email Piping > Closed ticket warning.
You will find settings to disable the option of replying to closed tickets in Support → Settings → Miscellaneous → Advanced → Allow reply to closed tickets.