[Exim] auditing bandwidth

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Hello-
I would like to use exim in a virtual host/reseller environment. I can not seem to find a way to log the amount of traffic (in MBs) that a particular domain has over a given period of time. It doesn't appear as if message size is logged in the logs. Has anyone solved this problem? I need to keep records of traffic for billing purposes.

Thanks for any assistance!
Jake
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