This article explains how to use Microsoft Outlook 2007/2010 to recover data from offline storage (OST) files.  Without Outlook, this procedure generally requires the purchase of third-party OST-to-PST conversion software.

When using “Cached Exchange Mode”, Outlook stores a local copy of your mailbox in an Offline Storage file with a .OST extension.  This file is located at C:Users<username>AppDataLocalMicrosoftOutlook and can be anywhere from 265KB to 5GB in size or more.

There may come a point where you need to recover your mail items from this file.  Outlook provides a built-in way to retrieve the content of these files, but the method is not easily visible from the surface.  The following steps illustrate a description from a user post on the WindowsReference.com forums.  Credit goes out to Sekoz! for outlining this method.  See here for more info.

Open the Mail applet from Control Panel.  Select the E-mail Accounts button.  Highlight the Exchange account and press Change.  In the Change Account window select the More Settings button.

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Go to the Advanced tab on the Microsoft Exchange (More Settings) window and untick Use Cached Exchange Mode.  Then hit Apply and click the Outlook Data File Settings button.

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In the Outlook Data File Settings window, click Disable Offline Use.  You’ll receive a message saying that Outlook will not be able to work with OST files.  Answer yes to the prompt.  This action will bring you back to the More Settings window.

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Click the Outlook Data File Settings button again and the Browse button becomes available.  Browse to your old OST file and click OK for the Outlook Data File settings window and the Microsoft Exchange (More Settings) window.

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Now you should be back to the Change Account window.  Place a check mark by Use Cached Exchange Mode and click Next | Finish.

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Now start Outlook.  You should immediately receive a message stating that Outlook is using an old copy of your Outlook data file (.ost).  Click OK here.  Let Outlook launch.  Do not restart Outlook at this point because this will force Outlook to create and use a new OST file.  When Outlook finishes launching you should be able to view all of the contents of your OST file, including mail, contacts, and calendar information.

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From here you should create your own archive/Personal Folders file and either click and drag the items from your old OST to the new PST.  You can also use the Archiving function to dump all the contents of your OST file to your new PST file. Don’t forget to revert back to your old OST using the same approach!

Hope this helps 🙂