[Exim] Routing after permanent failures

This problem is going to sound perverse, I can explain the history of it
is people are interested, but to cut to the facts (and you'll just have
to believe me when I say there is a reason why things are this way)(its
not a good reason - just a reason that is beyond my control).

At the moment I run exim 3.03 with a smart host for all external mail,
call it S.
At times S becomes VERY loaded, and takes for ever to deliver email, so
I'd like if possible to send email directly to the email addresses
concerned.
The problem is this, some addresses that I'd like to email to reject all
email from any machine that isn't the smart host; the only reliable way
to find out which machines do this is to try and deliver the email
directly to that machine; the problem is that these machines typically
give a permanent error message - and so the email is returned to the
sender. However if I could after such failures have the email passed
onto the smart host then it could deliver the email. The question is how
I force the email onto the smart host router after the email has
permanently failed after a direct delivery attempt; what I seem to want
is a way to make the router fail soft rather than giving a permanent
error, then the smart host router could cope with the email.

Any ideas ? (things like fallback_hosts for the smtp transport don't
work, as it does succeed in making a connection to the machine
concerned, it just doesn't deliver the email)

As an example of the kinda permannent errors I get with a direct
connection here is an example of a direct connection to the smtp port of
a machine that would reject direct email :-

telnet venus 25
Trying 193.60.196.15...
Connected to venus.dur.ac.uk.
Escape character is '^]'.
220 venus.dur.ac.uk ESMTP Sendmail 8.9.1/8.9.1; Wed, 15 Mar 2000
14:49:59 GMT
helo cpta7
250 venus.dur.ac.uk Hello cpta7.dur.ac.uk [129.234.9.56], pleased to
meet you
mail from: d.j.summers@???
550 e-mail may not be submitted directly to this host

However sending email to dph0djs%venus@icarus does get delivered (icarus
is the smart host).

Thanks in advance for any suggestions people have.

DAvid.