[exim] Exim as relay only - failover

We have an Exim server that only acts as a relay server and does not hold/manage recipient mailboxes. Can anyone suggest a (free but good) method for implementing failover for this box.

There needs to be a level of intelligence to determine if the server is up or down as a lot of the emails that go through this relay are generated by scripts or multifunction printers that don't hold emails for resending. I was thinking about DNS round-robin but this would not be suitable. Also some of the applications that generate mail have to use IP addresses and will not accept DNS names.

Regards,

Peter