Reject new ticket submission from email

We have added required custom fields based on ticket category. I would like to communicate with users by having them reply to an email for an existing ticket but reject new tickets being submitted by email because the custom fields can't be completed. An automatic reply back that I can customize would be great. I could add a link in the email that takes them to the ticking system from inside or outside the organization. We use jitbit for internal helpdesk only with a single organization. We don't develop or make a product so we don't deal with the general public.