[Exim] Problem with routers

Hi,

I have the following configuration for my routers: (exim 3.13 on
linux):

   outboundmail:
      driver = domainlist
      transport = remote_smtp
      route_list = "* 139.134.5.153 byname"
      pass_on_timeout = true

   lookuphost:
     driver = lookuphost
     transport = remote_smtp

end

We have 2 internet connections - ISDN and DSL. We want to router
all mail out through a smarthost of the ISP through our DSL box.
As DSL is pretty unreliable here is Australia, when the DSL link
is down, we want to perform direct delivery of mail through our
ISDN link. Our layout is as follows:

    ISP2            ISP2
    (ISDN)          (DSL)
     |               |
     |               |
     |               |
    firewall1-----firewall2
    (exim)
     |
     |
    LAN

Fireall1 has a static route to 139.134.5.153 through firewall2.
When our DSL link is down, exim complains there is no route to
host (as firewall2 no longer has a default route)

2001-05-24 06:42:03 152fSC-0002L9-00 139.134.5.153
[139.134.5.153]: No route to host

Ideally I would like exim to then pass onto the next router,
beging a lookuphost in this case. The documentation reports that
pass_on_timeout is for DNS lookups, but in this case I am using
an IP address (I have tried a few options, but this was the last
option I tried and obviously did not work).

Can someone please assist me is gettign this to work as desired?

Thanks,

Darren Mackay