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.
After the system reaches a limit, it does not block additional messages at SMTP time. Instead, the system accepts and then handles additional messages per the value of the The percentage of email messages (above the account’s hourly maximum) to queue and retry for delivery. setting in WHM’s Tweak Settings interface (WHM >> Home >> Server Configuration >> Tweak Settings).
To set limits for the whole server, or individual domains, please see the documentation below.