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Introduction to OST File, its Problems and Solutions

Author: Charanjeet Kaur | Updated on September 15, 2020 | Email Converter

Summary: This article will provide basic understanding of offline Outlook Data File (.ost) also called as Offline Storage Table (OST) file. It will also discuss several issues you may encounter while using an .ost file and how to fix them.

Table of Contents

Outlook uses data files (.ost and .pst) to store mailbox information on your computer. The data files help provide access to your synchronized or downloaded email messages, calendar entries, contacts, and other mail items without an internet connection. Now, let's discuss in detail about the Outlook OST file. 

Outlook Data File (.ost) – Overview

Exchange accounts, IMAP accounts, Office 365 accounts, and Outlook.com accounts use an offline Outlook Data File (.ost) to store synchronized copy of mailbox data on your computer. The file provides access to (previously downloaded) emails, calendar information, contacts, and other data, when connection to the mail server is interrupted.

Essentially, the OST file allows working offline, meaning you can do everything with the mailbox folders in Outlook without network or server connectivity. You can read and compose emails, create appointments, and add contacts. But, you cannot send or receive new messages until the connection gets restored.

When connection is established, changes made to the .ost file are automatically synchronized with the mail server.

Things You Need to Know when Using an OST File

When using an OST file in Outlook, keep these points into consideration:

  • You can back up offline folders (.ost) file by exporting it to PST file format.
  • OST files cannot be transferred between systems.
  • OST folders are automatically synchronized with the mail server. However, you may need to synchronize replicas of public folder and folders created by Outlook users.

OST vs PST

For better understanding of Outlook Offline Data File (.ost), it is imperative to know the difference between OST and PST file.

  • OST file stands for Offline Storage Table (.ost) file, while PST file stands for Personal Storage Table (.pst) file.
  • In Outlook 2016 and higher versions, OST file is used by several accounts (IMAP, Exchange, Office 365 and Outlook.com) for storing mailbox information. But, PST is used by a POP3 account only.
  • Unlike OST file, PST file is portable and can be copied from one system to the other.
  • Since.ost file is a cached copy of mailbox folders saved on the server, it isn't used as a backup file. This is because a fresh copy of the file can be downloaded from the server. However, a PST file can be used to backup or export mail items from any email account.
  •  OST files are meant for offline use and can be used by domains with limited network connectivity. But that is not the case with PST.

OST File Location

The OST file is usually located at any of the following locations:


drive:\Users\\AppData\Local\Microsoft\Outlook

drive:\Documents and Settings\\Local Settings\Application Data\Microsoft\Outlook


If you cannot find the .ost file, follow these steps to find the file:

Step 1:In Outlook, select File.


Select File tab

                                                                      Figure 1: Select File tab

Step 2: In the window that appears, choose Account Settings, and then click Account Settings.


Outlook Account Settings

                                                                      Figure 2: Outlook Account Settings

Step 3: In Account Settings window, click the Data Files tab and select an .ost file, and then click Open Folder Location.


Open OST File Location

                                                                           Figure 3: Open OST File Location

A dialog box displaying the default location of OST file will open.

You may also like to read: How to Change the Location of Outlook OST file?

OST File Problems

When using an OST file for Outlook, you may encounter any of the following issues:

  1. Orphaned .ost File
  2. When Exchange mailbox connectivity breaks with Outlook for a long time, or you've deleted an old Exchange server without properly backing up the OST file, the file may become orphaned.

    An orphaned offline folder (.ost) file means that it has become separate from an Outlook profile or an email account – that was used to create the file.

  3. Corrupt OST file

An OST file can become corrupt due to the following reasons:

  • Network connection failure or power failure during synchronization with Exchange server
  • Abnormal termination of Outlook
  • Bad sectors on hard drive on which OST file is stored
  • Large file size
  • Virus infection
  • Program or software conflicts

You may receive any of these errors when your .ost file turns corrupt:

  • Errors have been detected in the file xxx.ost.
  • Sending / Receiving reported error (0x800CCC0F): The connection to the server was interrupted.
  • Cannot start Microsoft Outlook.
  • 0x8004010F: Outlook data file cannot be accessed.

The Offline Folder (.ost) File becomes inaccessible when it gets orphaned or corrupt.

Troubleshooting OST File Problems

Here are some troubleshooting steps that can help you resolve OST file problems:

Step 1: Delete OST File and Re-create it

If an OST file in Outlook has turned corrupt or is inaccessible, try deleting the file and get a fresh copy by downloading it from the mail server. Once you know the location of Offline Folder (.ost) file, you can easily delete it. To delete the file, browse the location where it is stored. Find and select the .ost file with errors, and delete it. 

Once the deletion process is complete, open Outlook and it will automatically create a new .ost file. But, if it doesn't re-create the file, here's how you can do it:

  1. Open Outlook, and select File>Info>Account Settings.
  2. From Account Settings window, click the E-mail tab and double-click Microsoft Exchange
  3. Account Settings Window

                                                               Figure 4: Account Settings Window

  4. The Change Account window is displayed. Click More Settings
  5. Change Account Window

                                                                 Figure 5: Change Account Window

  6.  In Microsoft Exchange window, click Advanced>Outlook Data File Settings.
  7. Microsoft Exchange Advanced Settings

                                                              Figure 6: Microsoft Exchange Advanced Settings

  8. In Outlook Data File Settings, type the path of the OST file, and click OK.
  9. Outlook Data File Settings

                                                                     Figure 7: Outlook Data File Settings

  10. Click Finish.
  11. Step 2: Create a New Outlook Profile

    Sometimes, creating a new Outlook profile can help you resolve the OST file related issues.

    Steps to create a new profile are as follows:

    NOTE: If you want to use an existing .ost file, make sure to copy the existing file to a new location.

  1. Open Control Panel, and search for the Mail option.
  2. Control Panel Items

                                                                  Figure 8: Control Panel Items

  3. In Mail Setup window, choose Show Profiles.
  4. Mail Setup Window

                                                                    Figure 9: Mail Setup Window

  5.  In Mail window, click Add (as shown in the screenshot below).
  6. Use profile
                                                        Figure 10: Use profile 
  1. In New Profile window, type the name for your new email profile, and then hit OK.
  2. New Profile Window

                                                                   Figure 11: New Profile Window

  3. The Add Account window is displayed. Click Next.
  4. Add Account Window

                                                                      Figure 12: Add Account Window

  5. Once the account setup process is complete, choose Change account settings option, and then click Next.
  6. Change Account Settings

                                                                    Figure 13: Change Account Settings

  7. In the Server Settings box, choose More Settings.
  8. Server Settings Window

                                                                  Figure 14: Server Settings Window

  9. In the dialog box that appears, click the Advanced tab, and select Outlook Data File Settings.
  10. Outlook Data File Settings

                                                                 Figure 15: Outlook Data File Settings

  11.  In Outlook Data File Settings window, browse to the location where you wish to save the newly created .ost file, and click OK.
    Browse OST File

                                                                 Figure 16: Browse OST File

  12. If you're are using an existing .ost file, click Browse to select the copied .ost file, and then click Open.
  13. New Outlook Data File Window

                                                                  Figure 17: New Outlook Data File Window

  14. Select Finish to complete the setup for your email account.
  15. Now again open Control Panel, and then open Mail.
  16. 12.Choose Always use this profile option, select the new profile you've created, and then click OK.
  17. New Profile Added

                                                                  Figure 18: New Profile Added

    After successfully executing the steps, open Outlook again to recreate the .ost file for the account you want to repair.

    Step 3: Use Stellar Converter for OST Software

    Stellar Converter for OST software can help you extract data from an orphaned, inaccessible .ost file, by converting the file to .pst file format. OST to PST Converter can convert multiple OST files to PST format in a single process. 

    Why You Should Use Stellar Converter for OST?

    • Converts OST file to PST file format in three simple steps: 'Select', 'Convert' and 'Preview & Save'.
    • Converts mail items, while maintaining data integrity.
    • The Technician variant of the software allows to export OST file data directly to Office 365 & Live Exchange.
    • The Technician variant allows splitting and compacting resultant PST file.

    Read this: How to Convert OST to PST using Stellar Converter for OST software?

Frequently Asked Questions

Question. Can I open OST file without an Exchange account?

Answer. No.You cannot open an OST file without an Exchange account. However, Stellar Converter for OST can help you open the Offline Outlook Data files without an Exchange account.

Question. When trying to open an .ost file, am getting this error "The file.ost cannot be accessed because it has been configured for use with a different mailbox"? How can I fix this error?

Answer. You may encounter this error if you are trying to open the OST file on a different machine. But, if you are attempting to open an old OST file on the same machine used for creating that file, try these tips to fix the error:

  • Login into your machine by using the old username, used at the time of OST file creation(should be a cached login). Next,open Outlook and export the OST file to PST format.
  • Seek help from your Exchange Administrator. Ask the admin to export information from your "Old" Exchange mailbox.
  • Use a converter tool to convert OST file to PST file format.

Question. Can I manually convert my OST files to PST files?

Answer. Yes.You can use the Outlook built 'Import/Export' wizard to manually convert OST files to PSTs. But for manual conversion, you must be connected to Exchange server and have Outlook installed on your computer.

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