[exim] Message ID's / Message tracking

I have a number of Exim instances running in a load balanced environment
that pass the emails through a 3rd party Virus scanning appliance. However
the disadvantage of this is that when the appliance passes back the email, I
get a different Exim Message-ID generated. I know I can store the ID in an
X-Header, but is there any way of displaying this X-Header in a log file so
that I can track the message through the logs to see where in the platform
the message has got to.
Alternatively, is there anyway of making Exim use and ID that's in the
X-Header instead of generating a new one ?

I'm expecting 50-60 Million messages per month through the servers, so
processing time is of the essence.

Thanks..

-Andy-

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No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.323 / Virus Database: 267.6.5 - Release Date: 07/06/2005