[Exim] Alternative addresses for hosts

We run exim on out mail hubs and most systems at our University have MX
records pointing to our mail hubs. As a result of open relay problems
we're planning to prevent incoming traffic to port 25 from off campus
to any other systems.

We have two systems in one department and they have asked if we can arrange for
email to be sent to to the other if either is down (as would happen if they
each had MX records which included the other system). Is there a way of
getting exim to do this?

If the answer is in the manual or FAQ a pointer to where would be
appreciated.

thanks

Chris Bayliss
Information Services
The University of Birmingham