Personal Folder File (PAB, PST, OST)

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Microsoft Outlook uses the Personal Folder File (PFF) to store e-mails, appointments, tasks, contacts, notes, etc.

Three different types of the PFF are known:

  • The Personal Address Book (PAB), which contains the address book of contacts. These files have the extension .pab.
  • The Personal Storage Table (PST), which contains items like e-mails, appointments, tasks, notes, etc. and is used as current and archived mailbox files. These files have the extension .pst. The PST format is also referred to as the Personal Folder File (PFF) format.
  • The Offline Storage Table (OST), which contains items like e-mails, appointments, tasks, notes, etc. and is used as off line mailbox files in conjunction with Microsoft Exchange. These files have the extension .ost. The OST format is also referred to as the Offline Folder File (OFF) format.

The underlying file format of these files is the same of which the actual name is unknown but has been dubbed the Personal Folder File (PFF) format, because of its most common usage.

MIME types

The actual mime type of the PFF format is unspecified however some sources claim the following MIME types apply to this file format:

  • application/ (for PST files)

File signature

PFF has the following file signature:

hexadecimal: 21 42 44 4e


File types

There are a 32-bit and a 64-bit version of the PFF. These have the same file signature but can be identified by the version in the file header.


The PFF basically contains a hierarchy of items. The attributes of these items are defined by the Microsoft Outlook Message API (MAPI).


The PFF format allows the file to be encrypted. Two types of encryptions are currently known these are referred to as compressible and high encryption. The compressible encryption is a basic substitution cypher and the high encryption is a little more complex substitution cypher. From a cryptographic point of view this is more a way of obfuscation than a means to protect confidentiality.


PST or Personal Storage File is an open proprietary file format which stores email messages, contacts, calendar entries, tasks, and other items belonging to Microsoft software like MS Exchange Client, Microsoft Outlook, and Windows Messaging. The open file format is organized & structured by Microsoft who provide free specifications and free stable technology licensing.

PST file is a self-contained, stand-alone, structured binary file format which represents a message store comprising an arbitrary hierarchy of Folder objects holding Message objects holding attachment objects. The file format can also be known as; Personal Folder File or, Personal Address Book (.pab). When this file is functioning as a cache for Microsoft Outlook's Cached Exchange Mode feature, it can also be referred as; Off-line Storage Table (.ost), or Off-line Folder File.


PST file format is primarily related to “Outlook” application and comprises data of Outlook 97, 2000, 2003, or any higher version. Microsoft Exchange server stores messages and other data items on server. These items are stored locally on computer by MS Outlook application as offline storage table (.ost file). When this offline data is archived in Outlook application, messages are archived and stored as PST file.

PST file utilizes a fixed-block-based scheme for allocating memory. The file is enlarged by pre-defined sum of bytes and file preserves the information internally regarding allocated and non-allocated blocks. So when email messages are added to PST file, file size is adjusted accordingly by Outlook application. When any message is removed or deleted, file size remains unchanged marking space as unallocated where future items will be saved. Thus it has become easier to recover deleted data from PST files.

Types of Format

PST file format is currently of two types; ANSI and Unicode. Unicode PST is an advance format for PST and is available in Outlook 2003 and all above versions. It has storage capacity of 20 GB in Outlook 2007 and 50GB in Outlook 2010 & 2013. Whereas ANSI PST is legacy format which has 2GB storage space limitation and is part of Outlook 2002 and below versions.

PST Logical Architecture

PST file architecture is structured in three layers: NDB, LTP, and Messaging layer. The NDB (Node Database) layer comprises of database of nodes denoting the lower-level objects for storage function. It endures header, blocks, file allocation details, nodes, and BTrees (Node BTree & Block BTree). The LTP (Lists, Tables, and Properties) layer comprises higher-level notions comprises elements like Property Context (collection of properties) and Table Context (two-dimensional table). Messaging layer comprises of higher-level rules, it also connects the elements of LTP and NDB to be construed as high-level objects; Message objects, Folder objects, Attachment and Properties objects.

Outlook Offline Storage Table (OST)

Offline Storage Folders (OST) is the data file of MS Outlook. It is the replica of Exchange Server mailbox on local machine with .ost file extension and is automatically created when a user profile is configured with Exchange Server.

MAPI clients (like MS Outlook) can use OST file to store and access the folder information without being connected to the Server. It can be considered as snapshots of Server folders and this is the reason the OST file is also known as Slave Replica of Server. This file is automatically synced with the mailbox on server, provided there is unflawed connectivity between the client and the server.

Access Specification of OST File

An OST file can be accessed only through a MAPI profile that originally created it. If the OST file is not authenticated through its original profile, it will refuse to open. This happens because an OST file has an encrypted cookie that is created from the unique ID of the mailbox.

This unique ID can be only accessed when connection to the Server is made and this same encryption key gets saved into the registry settings of the profile. Every time Outlook is started to access an OST file, the encryption key at the server and the system registry is checked and if it matches at both the ends, access to the OST file is granted.

The common reason why OST file cannot be opened in Outlook is using a different profile to access the database. Also, deleting the mailbox or changing its location on Server will also result in OST file inaccessibility.

Location of OST File on Windows OS

Outlook Version Windows Version OST Location
Windows XP Outlook 2000/2003/2007 C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook\
Windows Vista/7/8 Outlook 2007 C:\Users\%username%\AppData\Local\Microsoft\Outlook\
Windows XP Outlook 2010 C:\Documents and Settings\%username%\My Documents\Outlook Files
Windows Vista/7/8 Outlook 2010/2013 C:\Users\%username%\Documents\Outlook Files

Also see

External Links