Overwriting Deleted Data During Backup
As with most applications, data that Exchange deletes is not actually removed from the disk. Although Exchange treats it as deleted data, it typically remains until it is overwritten by more recent data. If you want to make sure that deleted data is overwritten regularly, use the Zero out deleted database pages option. When this option is enabled, Exchange overwrites chunks of deleted data during the online backup process.
Important
Enabling the Zero out deleted database pages option can slow backup performance and increase the size of the database files. The option is turned off by default.