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What do you understand by OST File Format?

OST file format stands for offline storage table which gets created automatically though Microsoft Outlook offline folders feature. Offline storage table active when Exchange connectivity gets offline & synchronizes all updated email information when exchange turns online. In this file store, email messages, attachment, contacts, journals, notes, calendars etc. OST file format help users to work offline even the exchange server dismounted or down. OST is created in collaboration with MS exchange server that enables user to work with their messages & mail data. In offline case, when user try to send or receive email through your Microsoft outlook it asks user to create OST (Offline Storage Table) file because MS outlook get established properly.

If user are using Microsoft outlook as a client to an exchange server computer, user data will be stored on the server as usual. Sometimes this is not always the best configuration for users. The concepts of offline storage allows you to essentially carry on working in outlook even though you are not connected to your primary exchange server systems.

Offline folders make it possible to take a folder from server path, work with the contents of the folder when you are not connected to network & update the folder and its correlate server folder to make the contents of both identical. User can easily modify , add & delete the contents of an offline folder exactly as you can with a folder on a server. Offline folder are stored in the offline folder (.ost) file. This file is located on your computer hard drive and is available even when the network is down.

OST file format

OST File Location

For Outlook 2007

  • Windows XP or Windows Server 2003

c:\Documents and settings\username\local settings\Applications Data\Microsoft\outlook

  • Windows Vista
  • c:\User\user\AppData\local\Microsoft\outlook

For Outlook 2010
  • windows XP

c:\Documents and settings\username\local settings\Applications Data\Microsoft\outlook

  • windows Vista, Win7, Win8 and Win10
c: \Users\username\AppData\Local\Microsoft\Outlook

OST (Offline Storage Table) File Size Limit

  • The Default size of Microsoft outlook 2010/ 2013/2016 is 50GB for storing user data.
  • The Default size of Microsoft outlook 2003/2007 is 20 GB for storing user data.

Reasons to occur OST File Error

Reasons of OST File Corruption

Several reasons of OST file failure & the reasons may be related to hardware and software malfunctions.

Hardware Reasons

  • Due to power failure issue
  • Problem in networking
  • Crashed data storage device

Software Reasons

  • Due to malicious and virus attack
  • Corrupt file system recovery
  • File damaged

How to open OST files

When you add an exchange server account to your MS outlook profile, Cache exchange Mode is initiated by default.

To switch offline stoage for an exchange server account where it is not initially activated, do the following steps:

  • STEP 1: Right click the MS Outlook icon on the start menu, & then select properties.
  • STEP 2: When you see the mail setup box, click the email account button.
  • STEP 3: Then, Click the change.
  • STEP 4: Now click the more settings & then click the advanced tab
  • STEP 5: Click Offline folder file settings.
  • STEP 6: Now select the the file and path name and then click ok.
  • STEP 7: The last step is click Next >> then click finish.

List of Application that open OST file

  • Microsoft Outlook for Mac
  • Microsoft Outlook

OST File Security

Basically there is no way to password protect the offline folder .ost file used by outlook in cached exchange mode. The OST file is encrypted and uniquely associated with a particular mailbox.The OST file is stored should be an NTFS volume.


In this article, we talk about the concept of OST file format, list of application that open OST file. OST is a file format of Microsoft outlook. Also tell us about the way to open OST file, its location and reasons to occur ost file error etc.