[Exim] Load balancing and backup mail servers

Hi

I have a pair of servers providing access to a load-balanced array of web
machines. The 'outside' of the servers have two IPs (l1 and l2) respectively
with l0 being a floating address. Similarly, the 'inside' has two IPs (lb1
and lb2) with lb0 being a floating return address.

I would like to make server #1 (l1/lb1) into the main mail server and #2
(l2/lb2) into a secondary mail server.

Nothing difficult so far. However, because I'm running out of IPs in my
block, I would like to take out the l1 / l2 IPs and each load balancer would
bring up its external network for the floating address (l0) when it was in
control.

This would then mean that both mail servers would have the same IP address
(but only one would be exposed at any one time). The question is... can I
make the backup mail server forward to the main mail server after a
'failover' and 'resume' via the internal network?

Seem reasonable... or is it a stupid idea...

Thanks

Dirk