For this, you can make use of the outgoing email templates at the bottom of Configuration\E-mail Settings in your Lansweeper web console. You can use the Reply type State change (sent to user) for certain ticket types and only the 'Closed' state for example.
Alternatively, if you would make use of Automatic Ticket Closing, which also can be found under Configuration\E-mail Settings, you can setup two messages, one when you send a first follow up for when it is about to be closed and a second one for when it has been closed.