A new package from Dale Lovelace is MailScanner-MRTG which can produce a whole variety of graphs showing, for example:
I monitor MailScanner using MRTG in conjunction with this MRTG configuration and this script that is run by cron every hour on the maillogs. Due to some bugs in the version of MRTG I have got, I need to run the script every hour, though it produces the same results during each 24 hour period. Every night the day's mail logs are collected and put in 1 directory, which is where the script gets them from.
You will need to adjust the top of the script, and adjust the paths in the MRTG configuration, but hopefully it will give you enough to work from and save you having to write it all from scratch yourselves.