What is OST & How it Works?

If you are new to Exchange environment, you must know a very little about OST (Offline Storage Tables) files, also refers to as offline folders. Well, if you need to gain more information about the OST files, you are at the right place.

OST file is the replica of the Exchange mailbox stored locally and allows users to work offline when Cached Exchange Mode is enabled. OST serves a medium to work offline in the absence of reliable connectivity between Exchange Server and MS Outlook client machines.

When the connection to Exchange Server is retained, the OST file gets synchronized to Exchange mailbox and any edits made to the file are updated; probably scalable in the environment with unreliable and limited internet connectivity.

The two types of Outlook data files are OST and PST. Let's talk about PST and the difference between OST and PST files.

Difference between OST & PST Files

PST files are proprietary storage format for MS Outlook and stores user's mailbox data for IMAP, POP3 or other web based accounts. PST files or Personal Storage Tables stores email messages along with attachments, contacts, calendars and other mailbox components in an organized manner on all versions of MS Outlook client and Windows OS.

Outlook saves database information in PST files wherein Outlook does not have any connectivity with Exchange Server and runs independently. The PST files stores user data stored on local machine.

Location of OST Files

The location of the OST file usually depends on the version of MS Outlook client downloaded on the system. By default, OST data files are saved at the following location:

In Windows 10/8/7: "drive:\Users\user\AppData\Local\Microsoft\Outlook"

In Windows XP: "drive:\Documents and Settings\user\Local Settings\Application Data\Microsoft\Outlook"

Note: By default, OST files are created as 'Outlook.ost'.

To open OST file, it is necessary to exit MS Outlook and then follow the procedure as given below. Also note that OST files are the copies of Exchange mailbox; hence it is not essential to back up the file as new one can be created at any time.

  • After exiting MS Outlook, go to Control Panel and double click Mail to open it.
  • Click on E-mail tab and select Exchange account. Next, click on Change
  • Clear the checkbox corresponding to Use Cached Exchange Mode
  • Next, click on More Settings button
  • On Advanced tab, click Offline Folder File Settings
  • Click on Disable Offline Use, and then click Yes in the dialog box that appears
  • Note: If the Disable Offline Use is greyed out and you are unable to click on it, it indicates that Use Cached Exchange Mode checkbox is not cleared out.

  • Again click on Offline Folder File Settings and click OK
  • In then appeared dialog box, click Yes
  • Under Cached Exchange Mode Settings, select the checkbox available with Use Cached Exchange Mode
  • Then, click OK, click Next and then click on Finish
  • At last, close theAccount Settingsand Mail Setup dialog boxes by clicking the Close button.

Following the above mentioned procedure, you can open OST file in any version of MS Outlook; however, it is often seen that OST file turns corrupt due to unknown reasons.

What Makes OST Corrupt?

Just like any other data file, the OST files can turn corrupt at any instance of time. The major reason behind OST corruption is inappropriate synchronization issue that occurs due to bad internet connectivity. When OST files fails to sync with Exchange Server mailbox, they turn orphan and inaccessible. Other reasons such as virus infections, reaching stipulated storage limitations, etc. also adds to OST corruption.

Since OST files include crucial information such as business critical data and statistics, it is important to treat these files with utmost consideration to gain access to incorporated data. One easy way to restore access to OST mailbox data is via converting its contents to Outlook PST files automatically. OST Recovery software allow users recover data from corrupted OST files and convert the entire mailbox items to PST format, enabling access in all the versions of MS outlook.