This bug has been reported in a few different places, and I noticed it is happening to me also. Here are a couple of other unresolved instances:
http://www.virtualmin.com/ticket-tracker/bug?bug%5fnumber=425
http://www.virtualmin.com/forums/message-view?message_id=80702
In my case, I am using a default installation of Virtualmin Pro with Postfix on CentOS 4 (i386). I have confirmed that Bandwidth Monitoring is active for all servers, and the default mail log file is set correctly. However I do not see any e-mail bandwidth reported for my sites. They only show website and FTP traffic. If I change the display mode to "By Month", I do some "Mail received" traffic for a few months about a year ago (back when the server was first set up), but then it stopped and hasn't happened since. I don't recall making any changes to disable this, so perhaps something was broken in an update around June 2006?
BTW, as a related bug, the "By Month" display lists every month all the way back to 12/1969, even though that was way before Virtualmin was even a glimmer in Jamie or Joe's eyes. :-)