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OST File Format

What is an OST file?

An OST (Offline Storage Table) is an offline Outlook data file synchronized with an email server. It represents a local copy of the mailbox data that is accessible without an Internet connection. When connected to the Internet, any changes made to the .ost file will be synched.

For example, if your mail server is disconnected, you can access all previously downloaded messages, calendar entries, contacts, tasks, and notes. You can read and write new emails, create tasks and appointments, and add contacts. When the connection to the server is restored, new messages will be sent, and the mail received will be downloaded.

Outlook does not encrypt or provide password protection for OST files. To further protect your data, you can encrypt your drive.

What is OST file content?

The .ost file contains a complete copy of your Office 365, MS Exchange, or Outlook.com accounts:

  • messages, contacts, meetings, appointments, tasks, notes, documents, journal and calendar entries in different folders for the appropriate data types;
  • the properties and values of Outlook items and folder fields;
  • rules for handling incoming and outgoing messages, view settings, signatures, and search folders.

OST vs PST file

MS Outlook .ost and .pst data files are used to store Outlook account data. PST file is saved on the local computer. OST is a temporary file that syncs with the server. The file format depends on how the account was created. Outlook .ost file is generated when using IMAP, Microsoft 365, Exchange, and Outlook.com account types. If you are using the POP3 protocol, a .pst file will be created to store data.

When using the PST format, any changes to the mailbox don't show up on the mail server, and vice versa.

If your mail account data is stored in an OST data file, any changes you make to your mailbox are synchronized with the server. If you delete a contact on your computer, it will also be deleted on the mail server, and vice versa. If you send a message from your phone, you will find it in the Sent folder on your tablet.

When you set up your Outlook account on different devices via IMAP or MAPI protocols (i.e. using .ost file), then all your emails and settings are loaded automatically. The latest versions of Outlook, starting with Outlook 2016, are configured through these protocols by default.

The OST format for storing your mail does not provide additional data protection. To ensure security, the message database needs to be stored on the server and not on a local drive or external storage medium. E-mail servers are subject to increased reliability and security requirements.

If you do not enable Cached Exchange Mode during account setup, no OST file is created, and the data is downloaded directly from the server.

Where is the Outlook OST file?

The OST file location depends on which version of Outlook you are using.

  • Outlook 2007 and earlier on Windows XP:
    C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook
  • Outlook 2007 and earlier on Windows Vista, Windows 7, Windows 8, or Windows 10:
  • Outlook 2010 on Windows XP:
    C:\Users\%username%\My Documents\Outlook Files
  • Outlook 2010, Outlook 2013, Outlook 2016, Outlook 2019, and Microsoft 365 on Windows Vista, Windows 7, Windows 8, or Windows 10:
    C:\Users\%username%\Documents\Outlook Files

You can quickly check the Outlook OST file location as follows:

  1. go to File menu and click Account Settings,
  2. choose Account Settings... in the drop-down,
  3. follow the Data Files tab,
  4. select the required file and click the Open File Location... button.

To view the list .pst and .ost files, and other storage connected to your Outlook profile, use the Store and Data File Report utility.

OST/PST Outlook Data File Report

Opening OST file in Outlook

You can only open an OST file in the Outlook account in which it was created. OST files are bound to the email account and cannot be opened separately. You face additional limitations when opening the .ost file:

  • The OST may not store all the mailbox data, but only the data that the user needs. To download all the data from the server, you need to specify in the Outlook settings for what time you want to download the data to the local computer.
  • Folders marked Local Only are stored on the user's computer and are not synchronized with the mail server.
  • To open an old or orphaned OST in Outlook, convert it to PST using a special utility, and then open the resulting PST with no additional conditions.

How to move Outlook OST file?

OSTs are temporary files tied to specific Outlook account settings. When you move this account to another computer, you can't just import the OST file into Outlook. To import or backup OST, you need to convert it:

  • When moving the .ost file to an Outlook account configured through POP3, you need to convert OST to PST, transfer the PST to the new computer, and then import it to Outlook, for example, using the Import PST to Outlook utility. Otherwise, you will lose all your correspondence. The final PST can also be used to back up the data.
  • To save particular messages, contacts, events, and other items from your Outlook mailbox, it is convenient to use MSG format. Convert OST to MSG to save the separate items and transfer them to any Outlook account.
  • When importing an OST file to a mail client other than Outlook, you can convert OST to MBOX. The MBOX format is used by many email clients for data storage, import, and backup.
  • You can move messages from an OST data file to most other email applications through OST to EML conversion. The EML is also a common format for storing and transferring emails.

Outlook OST file size limit

The .ost files have a default size limit:

  • up to 20 GB in Outlook 2003 and Outlook 2007,
  • up to 50 GB starting with Outlook 2010 and later.

Once your mailbox reaches the default limit, you can no longer send, copy, or move messages in Outlook or receive new messages without clearing the mailbox or exporting part of the data to PST format. After the conversion, you can save the resulting PST and delete the exported data from the mail server. Then compress the OST to reduce its size.

How do I change the maximum size of an OST file?

Use the ReliefJet Outlook Tweaker tool to change the OST maximum size, and to enable or disable its automatic compression.

Outlook Tweaker

The increasing volume of data in mailboxes is often caused by large attachments in messages, tasks, notes, and other Outlook items. You can reduce OST size by reorganizing attachments:

  1. Pack attachments into ZIP archives.
  2. Save attachments in a separate folder, and replace attachments with links. Here, the attachments themselves are not stored in the .ost file, but separately on disk or network storage. Later you can restore attachments from links.
  3. Remove attachments sorting them by items, names, extensions, sizes, and message dates.
  4. Remove duplicate attachments.

To free up space in your mailbox and make your work more productive, you can delete or move to the archive of unused Outlook contacts, selecting them automatically according to the specified parameters.

How to restore OST file?

Sometimes you face situations where it is impossible to access data from OST. For example:

  • sudden power failure,
  • accidental deletion or corruption of .ost file,
  • virus attack,
  • Outlook sync problems (interrupted connection during synchronization),
  • file system error.

You may receive the following error messages:

  • Cannot open your default email folders. The information store could not be opened.
  • Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened.
  • Errors have been detected in the file %s.
  • Outlook. ost cannot be opened.
  • The file %s is in use and cannot be accessed.
  • Outlook is using an old copy of your Outlook Data file (.ost).
  • The file %s is not an Outlook data file (.ost).
  • Cannot open your default email folders. The Outlook .ost is not an offline folder file.
  • Outlook will not start. Missing .ost file.
  • Outlook cannot find the OST file.
  • Outlook. ost has reached its maximum size.
  • Send/receive the error. Outlook Data File cannot be accessed.
  • The operation failed. An object could not be found.
  • OST file failed to load.
  • Your Outlook data file cannot be configured.
  • The connection to the Microsoft Exchange Server is unavailable.
  • Synchronization error.

These errors occur when the data file is corrupted or the connection to the Exchange Server is lost for various reasons. You can try to recover data by converting OST to PST format.

You can also try to restore access to the data by re-syncing your mailbox with the server. To do this, you first need to delete the .ost file on your device. The next time you start Outlook, it will recreate the local .ost file containing the data saved during the last synchronization.

Important! To recreate the .ost file, Exchange Server or IMAP server must be in an operational state.

How to delete OST file?

It is easy to delete an Outlook .ost file if you know its location (see above). Before the delete operation, close Outlook, then go to the folder with the .ost file and delete it.

Important! You should make a backup in PST format before deleting the .ost file from Outlook. When deleting the .ost file, any offline changes will be lost.


How to move OST files to another computer?

You cannot simply copy the .ost file to access its content on another computer. Use the utility to convert OST to PST, and then open the PST in Outlook on the new computer.

How to backup Outlook OST files?

The easiest way to OST backup is to copy the OST and save it to the desired folder. To freely view the data later, convert OST to PST format. After conversion, you can open the resulting .pst in any Outlook account.

To open messages in other mail systems, convert OST to MBOX, the format used by most email clients.

What programs can import OST files into a readable format?

What tools to use to open the archived or orphaned OSTs depends on where you intend to view the information.

To view OST files in an Outlook account, you can convert them:

  • to PST files, to store and view the entire contents of the mailbox,
  • to MSG files, to store and view individual messages or other items.

The .pst and .msg files can then be easily opened or imported into your account.

To import data to other mail systems, convert it to a format supported by most email programs, such as:

  • MBOX, to view all messages from the mailbox,
  • EML, to view individual messages in folders.

How to convert OST to PST?

You can change the data file format with a few mouse clicks using the Convert OST to PST utility.

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