WHM should have a function that lists all email accounts and their statistics.
The problem I had earlier this week was that my server was attacked, and the attackers sent out random emails from different domains. The attacker was able to configure all error messages, warnings, and other communications to be sent to his own email address rather than the administrator's, ensuring that the administrator is unaware of what is going on until he performs a self-check.
Then I had to log into each cpanel individually, list all of the emails, and delete the ones that seemed suspect. Not only that, but all of the emails were eating up my hard space.
In WHM, there should be a "List all Emails" option that allows the administrator to see all of the emails from all of the cpanels that have been created, as well as information such as quota usage, peak hours of email received and sent, and so on.