[Exim] Limiting users to within a domain.

As part of some of our courses require students to use email, I'm
configuring an auto-responder to allow them to send to something like
auto@???, so that they can send and receive messages.

However, I'd also like to be able to restrict them so that if they try and
send an email to an address which is outside of our domain it is bounced,
with a report to the postmaster.