Any way to count monthly sent mails via mailman?

Hello people. I need to count how many mails have been sent by a given mailman list in one given month. Is there any script, console command or software to perform this?
I tried

Code:

exigrep 'newsletter@thegivendomain.com' /var/log/exim_mainlog|grep "<="

and

Code:

zcat /var/log/exim_mainlog.*.gz | grep "<= newsletter@thegivendomain.com" |grep "2010-08"|awk '{print $4}' | uniq | wc -l

but they are inaccurate and very cpu expensive.
The watched mailing lists are not setup to archive sent mails, so I have to figure out how to count the total mail sent. Any clue?