1. Login to the control panel.
2. Under Email section, click on the Email Disk Usage
3. Select the email account you want to view the disk usage
It will show you the number of messages and disk space used by it.
4. To free up/manager the usage, click on the manage option.
You can delete the messages by settings the required filter category under the Messages to delete section.
You will be able to check and maintain the disk usage as per email account easily in such ways.