@David Hall @Victor @Steve Giller
Thanks for your help, I got the rule from the bottom of this page:
https://wiki.hornbill.com/index.php/Email_Routing_Rules
I have app.email.routing.rules.unknownUsers.allow and app.email.routing.rules.unknownUser.sendRejectionEmail enabled but it doesn't seem to be doing the trick. I wanted to send a confirmation email to end users that are not recognised by the system to say that their request has been raised. For example, when I raise a request from my personal Gmail, the request goes through but I don't get a confirmation e-mail that a request has been raised. This works only for users that are already in the organisational data