It is possible there are some situations when Cisco Unity Servers run low on hard drive space. There are possibly different reasons for low hard drive space. One of main factors that cause this issue is possibly the Exchange Mail Stores and various Loggings, which take place on the Cisco Unity Server.
In order to conserve hard drive space on the Cisco Unity Server, it is necessary to relocate database and transaction log files to an alternate drive. This can be necessary if it is installed with the use of default drive locations.
Refer to Moving Exchange Message Stores and Transaction Logs to an Alternate Drive for more information.
In order to avoid potential loss of new messages and to resume logging and report generation, reclaim space on the hard disk. The Example Administrator account, which Cisco Unity creates during installation, serves as a default message recipient for the Unaddressed Messages and System Event Messages distribution lists.
If another subscriber is not assigned to these distribution lists, be sure to monitor the Example Administrator account and forward or delete the messages sent to this account as applicable. For subscribers on the Unaddressed Messages public distribution list, create a separate Inbox folder for returned messages. Then create a rule that automatically moves messages sent by the Cisco Unity Messaging System account from the Inbox to the Returned Messages folder.
Refer to these documents for more information on Cisco Unity:
Do not delete the Example Administrator unless you have assigned another subscriber to the Unaddressed Messages and System Event Messages distribution lists. After Cisco Unity installation, the only member of these distribution lists by default is the Example Administrator. If these distribution lists contain no members, messages sent to them are lost.