You will need an administrator mail account for this. To create an administrator mail account :
- Log into your Solid Control Panel.
- navigate to the mail domain you wish to create the administrator for.
- Create a new mail account.
- Navigate to the mail account you have created and check the 'Check to set account as Domain Administrator' check box and then save.
Navigate to your domain webmail login page.
Log in using the created administrator account.
Click on the Domain Reports tab in the top menu.
Click on the Disk Usage option in the left Menu.
This will provide a breakdown of the disk usage for each mail account created on the doamin name.