If you check the "Accounts List" in WHM and the "Disk Quota" is set to "Unlimited" despite the fact that the correct package is show then the account quota information stored in the server is corrupted.
You can fix this by "changing" the package for the account by selecting the same package and click update or
You can log into the server console via SSH and run the "fixquota" script.
Remember if you are using ConfigServer Mail Manage to override some of the default email setting, it will reset them to the default package limits and you will need to set the account as required.
