[Exim] Exim and router rules

Hi friends.

May by someone knows how to do something like this:

There is smart host and I know how to send all my non-local mail to it.
But I don't want to send there all
My mail, I want send mail to smart host only if my exim can not to
connect to server routed by the dns
Lookup.

E.g. I set two routers: DNS lookup and smarthost, after first one
message gets into smtp transport, transport
Can not to deliver message becouse my host behinde the firewall, but
there are many hosts whitch can be
Accessable directly whithout smart host. I can not to write list of them
becouse its too mutch :-(.

I hope there is another way: to try to connect by the DNS rules (MX
records tell my how to try to do this)
And if all tries fails to send message to my smart host.

Have You any ideas or may be HTTP links or some HOWTO ?