Alert if sending SPAM from PLESK

We have a linux server with PLESK 8.2.1 We have around 200 hosting plans there. Our problem is that sometimes some of our clients begin sending SPAM, causing all hosting plans to have problems (as the IP gets blocked). What we need is get alerted if someone is sending SPAM, that is, if a client sends X number of emails, or maybe if daily SMTP traffic is over a certain amount, then we receive an alert email. I don't know if there is PLESK component that could do this, or if something that should be built. Please specify and quote what you would do.