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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 and later 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.

For more information see:

Can you add to this? Do you know of any applications not already listed which will work with .ost files, view .ost files or open .ost files? Are you able to contribute any additional reference information or file format information about .ost files or have you spotted any errors or omissions? If so please let us know by emailing us at feedback@cryer.co.uk - Thank you.