A customer wants to create new Exchange mailbox or increase storage limit (disk space) for the existing one in POA Customer Control Panel. Control Panel says that available mail storage limit is 0, so customer cannot create new mailbox with non-zero mail storage limit or increase disk quota for the existing one. The screenshot below demonstrates the problem:
At the same time Customer Control Panel says that there is plenty of diskspace available in the subscription:
Other Exchange mailboxes or Public Folders utilized all the available mail storage space in a subscription.
During the new mailbox provisioning or changing storage limit for the existing mailbox POA ensures that total size for all Exchange mailboxes does not exceed the limit of the 'Diskspace' resource in subscription (or other resource based on the 'Diskspace' resource class). It does not matter if mailboxes are actually empty or total occupied diskspace in all mailboxes is less that the desired size of the new mailbox. In this case POA just uses configured mailbox limits, not actual disk space usage.
A customer can either decrease disk space limits for other mailboxes in the same subscription, or purchase additional disk space for their mailboxes.