When a machine's backups are about to exceed the storage space allowed by its subscription, you receive an e-mail notification with an alert. In addition, you can see this alert on the account management web page near the machine. This means you have to free some space for future backups. Or, you can consider increasing the storage quota. You may also want to set or edit the retention rule so that an overflow does not occur in future. Once the occupied space reaches the limit, the backups will cease to run.