[exim] failover to second outbound interface

I have a machine with two connections to the outside world. Normally,
mail goes out the default route unless an "interface" statement forces
it to the other connection. Can exim be told to try both interfaces
before it throws the message on the retry queue? The interface
statement can have multiple IP addresses, but the first valid address
will be used. Unfortunately, just because an interface is valid doesn't
mean it is actually working.

I could have a script change the machine's default route but I would
rather do this within exim if possible. I suspect I am missing
something simple.
--
Jay Rouman (jsr@??? jsr@???)