[Exim] Address rewriting

Hi,

I'm just setting up exim to be the router for a large number of
machines, this router needs to rewrite address so everything comes
from a common email domain. What I really want is a command that does
:-

*@local_domains $1@??? Ffr

That is any machine set up in local_domains appearing in the From
fields get rewritten. In fact all machine names are of the form
cpt*.dur.ac.uk ; so I thought that :-

*@cpt*.dur.ac.uk $1@??? Ffr

would work - however this seems to fail (I guess because its only half
the name before the dot that gets expanded).

Two typical addresses that need to be rewritten are :-

summers@???
summers@???

where both need to be rewritten as :-

summers@???

Does anyone have any idea of a quick snappy way of doing the rewriting
- or do I need to list the 30 odd machine names that could possibly
happen individually ?

Thanks in advance for any ideas that anyone has.

David Summers.