In addition to keeping an eye on your server's mail statistics, you should also pay attention to the Top E-Mail Relayers page. If one of your clients is sending out large quantities of e-mail, you'll probably see them on this list.
1) Find Email in the menu.
2) Go to View Relayers.
Here, you'll see a list of users that have relayed mail, in order by the number of messages sent.
3) Click on the user or the domain name to view more details.
This page shows the Relay Statistics for this user. You can see the time a message was sent, the ID of the message on the server, which e-mail address sent it, as well as the destination and the size of the e-mail.
This completes the tutorial. You now know how to view the Top E-mail Relayers in WHM.