Exchange Server and Outlook support forums are full of horror stories about users who have lost all the data in their mailbox because of a combination of factors. Perhaps the administrator accidentally deleted the mailbox or had to remove it and replace it to correct some corruption. Maybe there was a problem with the mailbox store backup or with the standby Exchange server, making it impossible to restore the mailbox to an earlier state. Even though Microsoft did not intend an offline folder (.ost) file--the local cache of mailbox data--to be a mailbox backup, the file does serve that purpose in some scenarios. But in the worst cases, the user--confident that all his or her data is in the .ost file and that he or she can synchronize the file with the server-–connects to a new mailbox, only to find that the .ost file won't work with the new mailbox and that the file is now orphaned, the data unrecoverable (unless the user resorts to expensive third-party tools).
The reason the .ost file no longer works is that Outlook encrypts each .ost file with a key that binds it tightly to a specific Exchange mailbox. When the user attempts to connect to a new mailbox, Outlook creates a new .ost file for that mailbox.
Microsoft Office Outlook 2003 addresses the need for a better recovery scenario by introducing a feature called Exchange Recovery Mode. This feature is part of the new Cached Exchange connectivity method that maintains a local cache of the user's mailbox on the user's hard disk. Chapter 6 of Microsoft's "Using Exchange Server 2003 Recovery Storage Groups" (see the URLs below) documents Exchange Recovery Mode and explains how to get a user quickly up and running with a blank but functional mailbox, while the administrator works to restore full access to the user's data by recovering it from a Recovery Storage Group. Although only Exchange Server 2003 supports the Recovery Storage Groups (see "Recovery Storage Groups Explained," May 2003, InstantDoc ID 39004), through which you can mount a copy of the mailbox database while the original is running, Exchange Recovery Mode in Outlook 2003 works with any version of Exchange.
Exchange Recovery Mode kicks in if Outlook starts with an existing Exchange profile but finds that the mailbox has changed. Under those circumstances, it offers the user two choices. The user can connect to the server to work online with the new mailbox, in which case the locally cached data will be unavailable. Or, the user can work offline, maintaining full access to the locally cached copy of the mailbox in the .ost file, but without the ability to send or receive new messages. The option that Outlook 2003 eliminates is working online with Outlook automatically building a new .ost file for the new mailbox and orphaning the old .ost file data. By getting rid of that option, which was the default behavior of earlier versions, Outlook 2003 protects the user from losing the data in the .ost file that is associated with the old mailbox.
If the user is working with a mailbox on Exchange 2000 Server or earlier and the administrator can't recover the original mailbox data, the user can start Outlook in offline mode and export all the data in the mailbox to a new Personal Folders (.pst) file. Then, the user can restart Outlook in online mode and create a new .ost file for use with Cached Exchange mode, as described in "Using Exchange Server 2003 Recovery Storage Groups." As a final step, the user restarts Outlook, which now has a new .ost file, and imports the .pst file data. Outlook 2003 will synchronize the local cache, including the imported items, with the server.
Of course, restoring the original mailbox is always a better option if at all possible. The limitation of recovering data from an .ost file is that the export doesn't include custom forms, views, and Rules Wizard rules, although methods are available to copy all those settings so that they can be restored later. In addition, any links between contacts and journal, appointment, mail, and other items are broken.
That's the beauty of the Recovery Storage Groups feature in Exchange 2003, which makes restoring the original mail database a faster and easier process. If you follow Microsoft's instructions for using the Recovery Storage Group to recover mailbox databases, Outlook 2003 will automatically return to its normal Cached Exchange mode operation the next time the user starts Outlook (after the restored mailbox becomes available).
"Using Exchange Server 2003 Recovery Storage Groups"
"Chapter 6: Recovering a Mailbox Database Using a Dial Tone Database"