What is an OST File?

Technical Description of .ost Outlook File Extension

Developer Microsoft
File Category Outlook Data File
Format Binary, Text
Supported platform Windows & Mac
File Category Email file

Understanding What Is an .ost File Extension?

OST file is the abridged form of Offline Storage Table. OST files are those files that get formed in an automatic fashion with Microsoft Outlook’s feature for offline folders.

These OST files tend to become active whenever your MS Exchange connectivity goes in the offline mode and then works towards synchronizing all of the updated emailing info and data whenever Exchange again gets in the online mode.

After this particular process, Microsoft Outlook application becomes online and all of the stored emails as well as other emailing information and data gets delivered to the respective clients right away.

So, precisely, we can say that, it is due to these OST files only that you are able to work in the offline mode too and your server too gets automatically synced. In some of the cases, however, where the internet connectivity is somewhat jittery and the network goes limited, precisely with unreliable internet connection, the creation of such OST files acts as a very excellent process for the Exchange users.

So, creating these OST files almost allows you, as a user, to work even in your server downtime circumstances as well without the need of halting the work operations.

What is in an OST File

In addition to storing synchronized data from the mail server so that work can continue uninterrupted, an OST file contains a local copy of mailbox data that enables Outlook users to access their emails and other items even while they are offline.

  • Emails: (Inbox, Sent Items, Drafts, Deleted Items)
  • Attachments to emails
  • Folder hierarchy and structure
  • Address book entries and contacts
  • Schedules for meetings and calendar events
  • Assignments and notes
  • Check the status of emails
  • Email information, including sender, subject, date, and time

How Does an OST File Work?

After knowing what is the ost file its time to understand how OST file work - while working offline, the Outlook email application tends to create an exact replica copy of your user mailbox on the MS Exchange Server that gets named as the OST or the offline storage table file, which are the files/ folders having OST as their file extension.

This mentioned OST file format is actually stored locally on your machine and is named so as offline folders files. These OST files are the ditto data copies of your folders that you must be having in your individual user mailboxes on the Exchange Server.

Each and every modification that is made when you work in the offline mode, is synched automatically whenever you get connected again to your MS Exchange Server.

Now, for using the Send and Receive commands, you can surely perform these send/receive functions; and even copy any changes you wish to make while still in the offline mode of work only. During this particular process, the permanently deleted email items will get erased from your copy at the Exchange server also.

Recommendation: Is there a need to convert Offline Outlook data file OST to PST then we recommend . It is a secure way to perform this critical task.

OST File Location

OST file in Windows XP, Outlook 2000/2003/2007:- C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook\

OST location in Windows Vista/7/8, Outlook 2007 :- C:\Users\%username%\AppData\Local\Microsoft\Outlook\

OST file location in Windows Vista/7/8, Outlook 2010/ 2013 :- C:\Users\%username%\Documents\Outlook Files

Benefits of Using OST File

  • OST file facilitate to work offline on your Outlook application. Users all across the world are using this particular method only for keeping their email work safe and secured
  • OST files offer great flexibility of working in the offline mode without making it mandatory to connect to the Exchange server always, which is a great relief as you can work unremittingly without the need of any continuous requirement of internet connectivity
  • With an OST file, your email data is always safe even when the server gets crashed unfortunately and even during any Exchange disaster situations

Types of OST Files

Outlook creates OST files in different scenarios depending on the account type now there are 3 major types you can find below:

Exchange OST File

This is most popular and used in corporate environments with Microsoft Exchange Server allowing offline access and syncs automatically when the server becomes available.

IMAP OST File

This is Created for IMAP based email accounts like Gmail or Yahoo and it stores a local cache of server emails but becomes inaccessible if the account is removed.

Microsoft 365/Outlook.com OST

This is simply used with cloud based Microsoft 365 accounts. These OST files rely on modern authentication and can become orphaned if the Outlook profile or tenant changes.

Difference Between OST and PST File

Feature OST File PST File
Server Dependency Yes No
Offline Access Yes Yes
Backup Friendly No Yes
Portable No Yes
Profile Dependent Yes No
Risk of Data Loss High Low

Check Common Scenarios Where OST File Becomes Inaccessible

  • Exchange account deleted or disabled
  • Outlook profile removed or recreated
  • System crash or OS corruption
  • Domain or tenant migration
  • Outlook reinstallation
  • Encryption mismatch error
  • Sync failure after Outlook update

Is OST File a Backup?

No. an OST file should not be treated as a backup as it is encrypted and tied to a specific Outlook profile so if the associated account is removed the OST file cannot be accessed directly.

Can OST File Be Opened Without Outlook?

No - OST files cannot be opened directly without Outlook because they are encrypted and profile dependent. Microsoft does not provide any native OST viewer.

So if you want to access data from an inaccessible OST file then conversion to PST is the safest option.

Reasons of OST File Corruption

  • Sudden shutdown of Outlook
  • If there is a network interruption during sync
  • Oversized OST file
  • Antivirus conflicts
  • And Faulty Outlook add ins

When to Convert OST to PST

  • When exchange server is permanently unavailable
  • If your mailbox has been deleted
  • If you need to recreate Outlook profile
  • If data backup is required
  • Office 365 migration is planned

Security/Encryption in OST Files

Outlook profile credentials are used to encrypt OST files, preventing unwanted access but making recovery hard in the event that your profile is lost.

What are the Limitations of OST File

Final Verdict

Since OST files are not meant for long term storage or backup as they are made to offer brief offline access to mailbox contents. However OST files function effectively as long as your Outlook profile and mail server are operational. However your data stored in OST files is highly vulnerable if the profile or server is inaccessible.Converting OST to PST is the most reliable option for data security, backup, or transfer.