[exim] Retry permanent delivery errors ( to a different host…

Hi,
This may sound a bit odd, but their is a reason...

I want to configure exim to retry a permanent delivery failiure (ie a
5xx ) response, before it bounces the mail back to the user.

The senario is this...

exim is delivering via MX, but some hosts dont want to accept mail from
the exim server due to IP address restrictions. So rather then
reconfiguring exim every time this happens, I want it to rety a smart host.

The reason for this,

1. The smart host I use is unrelyable and has been known to drop emails.
2. Many MX hosts are rejecting more and more IP ranges, incliding 100%
legitimate static IP ranges. They rightly respond with a 5xx response,
but the response is only valid for the route taken, not the act of
delivery, ie if it went via a smart host it would deliver.

I've spent several hours on google, and scanned the source code but cant
see a way of doing it. Has anyone got any ideas ?

Ian