[exim] Configuration to first try direct SMTP and if we get …

I think subject is self-explanatory,
Really often machines behind an ADSL line with dinamically assigned IP
address have issues sending mail to some mail servers (RBL blocked for
having Dynamic IP address or popular ISP).
We have some hosts in a datacenter that have no problem sending mail but
traffic cost us money...
I want to have on this datacenter some host that acts as an smarthost for
hosts behind ADSL lines but only when it is necessary, so I can save traffic
on the datacenter

The question is: can I try to deliver directly and if I get a permanent error
(probably relaying denied) send this message to my smarthost? How can I
configure this in exim.conf? I'm now using exim3 but I woundn't mind
upgrading to exim4.

Thanks,
--
Agustí Rivero
xarxes@???
Telf. 902 36 14 84

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