>> certain customers may be using their accounts to send spam through the servers.
Well, let's see what kind of spam control methods you have in place...
I'd start by checking out rblsmtpd to stop spam:
http://www.linuxquestions.org/questi...light=rblsmtpd
If you're using this, it will help to stop a great deal of spam at the door.
You should also check to see if you have qmail-scanner installed. At the prompt, enter...
ls -l /var/qmail/bin/qmail-scanner-queue.pl
-rwsr-xr-x 1 qscand qscand 109384 Aug 25 12:09 /var/qmail/bin/qmail-scanner-queue.pl
If you have it, that's good. If not, I think I woudl recommend installing a qmail server with a lot more features. These days, a standard vanilla installation of qmail just ins't enough to maintain a production server for a lot of users. Check the link in my signature for a great qmail howto that has all the bells and whistles you need to run a good system. Or you can also try qmailrocks.org for a good howto. This will get you some good spam control methods in palce (qmail-scanner + ClamAV + Spamassassin) which helps a great deal. Once you have all of these in place, you can modify your installation to begin scanning your OUTGOING mail with the same tools. This will help you to more easily identify and tag who among your customers is behaving improperly. You'll also have some good logging tools (qmailAnalog + qmail MRTG) so you can see how much outgoing bandwidth you're using. This will also help you identify who on your local network is causing you trouble.