In order to prevent a compromised account sending large amounts of spam and damage the reputation of the server, it can help to limit the mails a domain can send per hour. This article covers how that can be done.
1) Log into WHM and navigate to the following location.
Home » Account Functions » Modify an Account
2) Select the account you'd like to modify and click the "Modify" button.
3) Set "Maximum Hourly Email by Domain Relayed " to the hourly emails you'd like to allow domains under this account to be able to send. A value of 0 (zero) will be interpreted as no limit.
4) Click the "Save" button.
To set limits for the whole server, or individual domains, please see the documentation below.