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Beware of OST problems when rebuilding Exchange stores

Sometimes Exchange admins have no other choice but to rebuild Exchange stores from scratch. But doing so can create unexpected consequences if Outlook clients are using OST files.

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Sometimes, in the wake of a disaster, Exchange administrators have no other choice but to rebuild their Exchange stores completely from scratch. This could be because no suitable backup is available, or (more rarely) because incremental changes in versions of Exchange have made it impossible to use earlier iterations of the data store.

Unfortunately, building a whole new set of Exchange stores can create unexpected consequences if Outlook users are using OSTs, or offline folder files.

OSTs are encrypted, and the encryption keys for OSTs are kept in the Information Store. If the store is reset and an Outlook user with an OST keyed to the old store logs in, a new key is generated and the data in the old OSTs is rendered unreadable.

This behavior has been remedied in Exchange Server 2003 -- it doesn't automatically generate a new key in such a situation. But many companies are still on older Exchange installations, so they're still vulnerable to this problem.

One workaround is to force clients to connect only in offline mode until they can safely export the data in their OST files (assuming the data in those OSTs is important). The easiest way to do this is just to disconnect the workstation in question from the network, open Outlook and perform whatever work is needed on the OST file from there.

Another, more roundabout way, is to create an additional user profile in Outlook that accesses the same OST file, but is configured exclusively for offline use.

Note that if you're using Exmerge to recover mailbox data, it can't recover OST encryption keys, or most other mailbox metadata either. The only reliable way to get such data back is with an actual backup of the Exchange database that has metadata in it.

About the author: Serdar Yegulalp is editor of the Windows 2000 Power Users Newsletter and a regular contributor to


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