would be nice if you could just have a trigger that sends a nice reply and closes the ticket.
Like: this is an employee only support service, please visit our web here… or there…
I am not sure triggers can reply. And it seems possible to match tickets not coming from your organization, but organizations seems limited to 1 domain.
having multi-domain organizations (which is common) and ability to reply from a trigger perhaps could help.
If filtering is no option for you of does not fit your needs.
I would recommend to configure this by a Mailserver.
This can easily be done in several ways.
One way would be using black and white list. Only you chosen domains are on the whitelist, everything else by default on the blacklist and can be rejected.
Another example using office365. Import Mails only from a mailfolder instead of the inbox folder itself. Only mails from a specific domain or with a Ticket-ID in the subject are moved to the subfolder and will be processed by zammad.