How to monitor and get notify which VPS within one node is sending mass emails ?

How to monitor and get notify which VPS within one node is sending mass emails (spam) ?

Actually I just got one node with CentoS OS and few KVM VPS clients, some of them are sending bulk emails and datacenter keep sending warnings that my server will be suspended if thing continue like this.

I need some how to automatically get warnings if one VPS have emails stuck in queue, this way I will immediately send a warning to client and suspend/delete VPS if he continues.

http://ift.tt/1dvHQ0Z