[Exim] Rewriting recipient addresses

Hi all:

One of my clients recently upgraded their Microsoft Small
Business hardware, using a fresh reinstall (which halved
the RAM usage, using the exact same software; go figure).
Because we didn't follow the anally precise 40-step process
for moving user mailboxes from the old Exchange Server to
the new Exchange Server, when users reply-all to messages
received on the old Exchange that were migrated to the new
Exchange, Exchange puts out a munged CC header:

<IMCEAEX-_O=THADDEUS+20COMPUTING_OU=THADDEUSNT_CN=RECIPIENTS_CN=HAL@???>

Predictably, when the a recipient of the reply-all does his
or her own reply all to the message, Exchange Server bounces
it back, because its looking for a corresponding X.400
address which of course is impossible to match. Of course,
the client is very concerned that his credibility is taking
a hit, even though the From: header matches the intended
recipient of the munged header, and to the intended recipient
does indeed ge the response.

So now I have to get their exim gateway to unmunge incoming
headers like

<IMCEAEX-_O=THADDEUS+20COMPUTING_OU=THADDEUSNT_CN=RECIPIENTS_CN=HAL@???>

to be like

hal@???

Is there any way to do this? I looked in the FAQ, everything
seemed to apply to local_domains, not relay_domains.

Thanks,

--Christopher