Can I set up alerts when users send too much email?

[JerkyChew]

We're having more and more problems with users sending large amounts of emails, and getting our IPs blacklisted with various RBLs and AOL. Whether it's intentional, or an effect of some kind of mass-mailing worm on their clients, it's really starting to be a problem.

Is there a way I can set up some kind of alerting system that will let me know when a user is sending a crazy amount of emails out? Or as an alternative, is there a decent log analyzer that I can use to see who's sending what to whom? so I can see who's sending the most emails, server-wide? Our Plesk 7.5.3 box hosts about 300 domains at the moment.

Thanks!