I can tell you from experience that backing up Exchange mailboxes with BE is fraught with problems. There are many things that will cause BE to fail on a mailbox such as the associated AD account being disabled, attachments that can't be accessed for whatever reason when single instance backups are enabled, etc, etc. Not to mention the amount of time it takes to actually do a mailbox backup.
My solution? Don't do mailbox backups...only backup the information store. This has a couple advantages with really no disadvantages. First, its much faster. Second, its easier (and faster) to restore the storage group you need to a recovery storage group and then use exmerge.exe to extract the mailbox you need to a PST file and mount it as a personal folder to recover whatever you need. I haven't yet found or thought of an instance where this method isn't sufficent.
http://seer.support.veritas.com/docs/264815.htm will show you how to use a RSG to do the restore.
Hope this helps.
