An administrator wanted to know how to get monthly stats for the internal email traffic and message sizes.
You can use a PowerShell to get traffic and message stats, and you don't even have to write your own script. There is a ready-to-use PowerShell script at Exchange 2007/2010 Email stats.
This was written to get "yesterday's" stats for each user, both internal and external, but you can consolidate the daily stats into monthly stats. See the Q &A at the above site for more information.
More PowerShell scripts for Exchange server are available in the TechNet Galley