Frojon Banwell

You or your Admin needs to verify they are indeed "follow-up", Nelson.  If not, then your issue would be something else.  If they are follow-up you might be able to train the users to not choose an auto-filled email address to your Zendesk with a ticket number in the address.  The training has worked for many of our users and we get less now.

Looking at this again and finding that "Changed from" is an option in Triggers rather than Automations,  But, still does not make sense since a new submission to support+123456 (with 123456 being a closed ticket) will not change the status of a closed ticket.  It will open a new ticket referencing the old ticket.

Garry's solution does not seem possible in my setup.  There is no Status changed from closed, and an email generated from a closed ticket opens a new ticket.