Logging throughput per domain for charging purposes.

[jeremy13]

I run a pair of Sendmail servers, latest version. A part of their purpose is to relay mail as a secondary MX for a number of other domains. I now need to advise a representative of each domain on their throughput, partly so that I can levy a charge on them for excessive use.

Is anyone aware of a log analysis method which will report the throughput handled by each server for each domain?

Thanks in advance.