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.

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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.
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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.

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This completes the tutorial. You now know how to view the Top E-mail Relayers in WHM.