Microsoft Exchange Server now writes event messages that provide an estimate of the free space in the public and private information stores. The free space is the amount of disk storage that would be returned to the file system if you ran an offline defragmentation of the information store.
The free space reported is the amount of storage in completely empty pages in the information store. Because there may also be partially empty pages, the reported value is a conservative estimate. Free space in the information store is not usually a problem and is managed automatically by the online defragmentation process. The free space message is generated during this online defragmentation.
The free space message is written to the application event log with the Event ID 1221. The Source value in the message indicates if the message is for the public information store or the private information store.