[Exim] changing from exim3 to exim4

I have a large exim3 config file which took a lot of effort to develop when I moved
over to using exim 12 months ago. I intend to move to exim4 as soon as possible.
However, e-mail is an absolutely essential service and we operate on the principle
of doing everything possible to prevent any disruption to that service. It follows
that it is going to be a real problem changing over. Testing the exim4 config is
very difficult because I can't just put it up and see what happens.

I have been running through a lot of "exim -C newconfig -d -bt" tests to see that I
get the same as before and have noticed a difference. I have a router (used to be a
director) which does a data = <lookup username for this local_part> and has no_more
set. For non-existent local_parts the no_more made it skip to the end of the
directors and fail with 'unknown local-part "xxxxx" in domain "ex.ac.uk"'. With
exim4 I now get 'Unroutable address' as the failure. If this means that the
end-user is going to get a non-delivery report with that explanation then I am not
at all happy. The latter message is very misleading because I would expect people
to interpret it as unroutable in networking terms, which is not the problem at all.

How do I get back to the 'unknown local_part' type of failure?

Phil.
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Phil Chambers (postmaster@???)
University of Exeter