If my customer created 2 mailboxes (3GB mailbox size for each) he is already allocating 6GB of disk space despite the fact that the subsciption is only 3GB.
The same situation is for disk usage. My customers are able to overuse disk space. How can I restrict the over usage?
The overuse policy does not apply to the limits set on size of mailboxes.
For each subscription, the following types of allocated resources are listed in the Panel at the
Account tab > Subscription name > Resources tab
Disk space. This is the total amount of disk space allocated to your account with subscription. This amount includes all data related to your websites, e-mail accounts, applications, backups, and log files.
Mailbox size. This is the amount of disk space that can be occupied by a mailbox.
Overuse policy defines what happens if the subscription's usage of disk space and traffic reaches limit values.
If you select "Overuse is not allowed" then a subscription is automatically suspended if the resource usage exceeds the limit values. If you select "Overuse is allowed (not recommended)" then it will allow the subscriber to use more resources than initially provided with the subscription.
Note: The overuse policy does not apply to the limits set on size of mailboxes. Therefore, even if you enable overuse, be sure to allocate enough disk space to mailboxes.
Set the proper overuse policy you need in accordance of documantation:
You can also enable 'Hard disk quota'. See the article "What is the difference between "Domain disk space" and "Hard disk quota"?"