To change the maximum number of email a domain can send per hour, follow these steps
NOTE: This changes it for ALL domains on the server
1) Login to the WHM for the server.
2) Click on Tweak Settings under Server Configuration
3) Change “The maximum each domain can send out per hour (0 is unlimited)” under the mail category to the correct setting
4) Click on “Save” at the bottom of the page.
For individual accounts:
You may further refine the above setting with the Home >> Packages features to add and edit packages or the Home >> Account Functions >> Modify an Account feature to edit this setting for individual cPanel accounts.