Internal mail reports (log)

Hi all,
The company I work for would like to have internal mail flow reports.
They are interested in the number of emails sent internally at certain times (hours, days - eg at weekends) by specific people or groups (some kind of activity reports).
MTA provides Message Logging API but last info is from 1997.
Obtaining similar information about exchanging external emails from GWIA logs is very easy, but I do not see in any of the logs the possibility of obtaining similar internal mail information.
Any ideas ?