Microsoft Outlook is a Personal Information Manager that comes as a part of the Microsoft Office suite. Being one of the most popularly used e-mail clients, it not only enables the users to view and store their e-mails but also includes other features such as a task manager, a calendar, a contact manager, notes, a journal, and web browsing.

In order to store the data, Outlook Incorporates two data storage files:

  • PST (*.pst) – Personal Storage Table
  • OST (*.ost) – Offline Storage Table
These two file formats behave as the data store house for Microsoft Outlook, where *.pst and *.ost are the file extensions used by Outlook to create a data file for a particular account configured in Outlook. Usually a ‘*.pst’ is used to store the data for Outlook. In case Outlook is configured with to work with an Exchange server account, its data items are sent and saved to the mail server. In order to work with the messages even when we not actually connected to the Exchange Server another data storage file is the ‘*.ost’ file.

What actually is Outlook Data File?

An Outlook data file is an open proprietary file format that acts as Outlook’s main repository for all the email messages as well as other items such as the Calendar, Tasks, Contacts, etc.
While computing, a PST or a Personal storage table file is a data storage file for Microsoft Outlook. 
When is working with the Cached Exchange Mode, it is called an OST or an Off-line Storage Table.

The main Difference among these two file types is that a PST file is used for a POP3 or IMAP connection (not in case of Outlook 2013) where as an OST is created when we want to work offline with an Exchange account using the cached exchange mode. This type of data file is also created when we create an account set up using Outlook Connector for Outook.com.

PST – Personal Storage Table

This data file is most commonly used by the home users and small organizations in order to store Outlook information. In order to connect to the internet the home users usually use an Internet Service Provider (ISP). Usually the account type is referred to by their Internet Protocol used for the account setup, i.e. POP3 & IMAP. Another account type that is similar to the IMAP is the HTTP or a web-based account.

The PST file can also be used for moving data items from the server mailbox and archiving them or keeping them on the local machine. This can be of benefit by freeing up the mailbox storage space. If configured, any new mail or item would be delivered to the PST data file, although it can have several disadvantages, including not being able to work on the mails when we are offline or disconnected from the network.
#Note: In case a PST is accessed from a shared network it is prone to data loss.

OST – Offline Storage Table

The Offline Folders are the exact replica of the Exchange Server mailbox that is stored on the local machine. This enables the user to work offline with the contents of the folder in case we are not connected to the Exchange Server. As soon as the connection is established updates made local machine are copied to the data at the server side. This process is called Synchronizing.

The OST file have their own benifits:

  1. Beneficial where the internet connectivity in either limited or unreliable.
  2. Does not require to be connected to the Exchange Server on a constant basis.
  3. Although time consuming but in case the server crashes the data can be retrieved from the OST file. In order to do this the user has to convert the OST to PST. This should be done as the data could be fetched from a PST even if the respective profile is deleted.


Outlook Data File Storage Structure

An Outlook data file incorporates a B-tree storage structure that includes 512 nodes and leaves in it. Having a B-tree data storage structure enables the data file to sort the data and allow the following features:

  • Searches
  • Sequential access, insertion and deletion of data
  • Logarithmic time
But unlike the self-balancing search trees, a B–tree storage structure is an optimized structure for a system that enables large blocks of data to read as well as write. This structure is commonly used to manage large databases and file systems.

ANSI Character Encoded 

Until Outlook 2003, the character encoding used by Microsoft for Outlook was the ANSI format. ANSI can be defined as the Windows code page incorporated to run an application that is native non-Unicode using the G.U.I. on a Windows system. An ANSI character encoding Outlook data file has the maximum file size limit of 2GB (2 31 bytes).

2 GB File Size Limitation in an ANSI PST Data File: As soon as the file size exceeds 2 GB, it gives an error message and is prone to becoming corrupted. Although the ANSI format is suppressed it is still being supported in Outlook 2007 and its previous versions, incorporating IMAP4 and HTTP accounts.

UNICODE Character Encoding


Beginning with Outlook 2003 a new encoding format was introduced for the Outlook data files: UNICODE. UNICODE can be classified as a standard for the computing industry that maintains consistency in the encoding, representation and handling of text used most frequently in the world of writing systems. The UNICODE defines a code space of 1,114,112 code points in the range 0 hex to 10FFFF hex. Apart from this the UNICODE character encoding uses a 64–bit pointer rather than 32–bit pointer that results in overcoming the 2GB limit of the ANSI encoding.

Data File Location on the local computer


The fastest and the easiest way to find the location where the Outlook Data File is saved is:

  1. In Outlook, go to the File tab.
  2. Click Account Settings
  3. On the Data Files tab, select a profile name, and click Open Folder Location.

Default location for a *.pst data file
  • Windows 7/Vista
%appdata%\Local\Microsoft\ Outlook
  • Windows XP
%Documents and Settings%\user\Local Settings\Application Data\Microsoft\Outlook
Default location for a *.ost data file
  • Windows 7/Vista
%AppData%\Local\Microsoft\ Outlook
  • Windows XP
%Documents and Settings%\user\Loca
l Settings\Application Data\Microsoft\Outlook

Comments (1)

Thanks for sharing such a useful information.

Have a question about something in this article? You can receive help directly from the article author. Sign up for a free trial to get started.