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.OST File Extension

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File TypeOutlook Offline Folder File

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CategoryMisc Files


This file is saved in a binary format, which requires a specific program to read its contents.


Stores e-mail messages and other data saved by Microsoft Outlook (2005 or later) from an Exchange server; similar to a Personal Folder (.PST) File, but supports larger file sizes.

More Information

OST files are created when the "Use Offline Folders" option is selected in Outlook and are stored on the local hard drive instead of mail server.

Programs that open OST files
Microsoft Outlook 2016
Remo Convert OST to PST
OEMailRecovery.com Outlook Recovery Toolbox
Updated 9/18/2015