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.ost file extension

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.ost

Microsoft Exchange Offline Storage file for e-mails (and folders). This allows emails to be viewed (and replied to etc) while disconnected from the Exchange server. The file contains a copy of the Exchange mailbox, thereby allowing offline viewing.

The default location used by Microsoft Outlook for .ost files is %UserProfile%\Local Settings\Application Data\Microsoft\Outlook, but this can be changed in the registry. The default name is outlook.ost.

If the file is deleted then Outlook will recreate the file automatically. This is true for Outlook 2003, but is not known whether this is true for other versions. Whilst Outlook will recreate the file, this can be a lengthy process because Outlook needs to pull down each of the folders and emails from the Exchange server.

Relevant links:

www.caelo.com/kb/T1120.php - Article on dealing with corruption issues with OST files.

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