The following additional limitations exist for snapshot operations performed with Exchange 2007 and earlier:
Snapshot backups are supported only for the full Information Store, or storage groups. The entire Information Store or storage group must be selected for backup. (However, you can select individual databases for restore.)
Snapshot backups are not supported for individual Exchange mailboxes or public folders.
(Exchange 2003, pre-Service Pack 1) Differential and cumulative backups are not supported.
You cannot restore snapshot backups and streaming or MAPI backups in the same restore job.
An incremental backup must be of the same type (streaming or snapshot) as the last full backup.
More Information
Configuration requirements for snapshot backups of Exchange Server