You can use System.HostName.
I concat a literal and the host name to the subject line:
Email.Send ("Admin Email Address Here", String.Concat ("Site Review Request: ", System.HostName), User-Defined.notificationMessage)<Site Review Requests Email>
So it sounds like I'd have to edit every one of the rules that sends mail then? That will work.
It's unfortunate that this can't be done centrally in the "Email Notification" settings.
Sorry. I can't see any other way.