After our previous blog post on .ost files and Complete PC Backup, a customer contacted us about how he loses column settings, sort order, and synchronization filters if he manually deletes the .ost file. I asked Alon Brown, a Development Lead for Outlook, to respond:

View settings should generally be preserved because they are stored in the .ost but are synchronized to the server. However, there can be specific timing situations in which changes will be lost. The synch filter settings are .ost-specific because it is reasonable that a user would want different synch filters on different .osts (e.g., an .ost on their desktop machine might have no synch filters while one on a laptop with limited bandwidth might have restrictive filters). Other send/receive settings should be stored outside of the .ost and, as long as a new profile is not created, should survive the deletion of the .ost.

More generally, the problem with backing up and restoring the .ost is that it will not behave the way that users expect. Specifically, if an .ost "moves backwards in time" (as it would in the case where a backup was made, more changes were synchronized, and then the backup was restored), Outlook refuses to perform any further synchronization with the server because it cannot guarantee that the user's data will not be corrupted in various ways. Therefore, because the restore operation will almost certainly fail (unless you're in a scenario in which you had significant offline changes that you want to load up, copy to a PST, recreate your .ost, and then copy them up to the server), there is no value in maintaining an .ost backup.

--Alon