Outlook PST Recovery

In Microsoft Exchange Server, the messages, the calendar, and other data items are delivered to and stored on the serverMicrosoft Outlook stores these items in a personal-storage-table (.pst) or off-line-storage-table (.ost) files that are located on the local computer. Most commonly, the .pst files are used to store archived items and the files to maintain off-line availability of the items. This is an essential feature of Microsoft Outlook.

The size of these files no longer counts against the size of the mailbox used; by moving files from a server mailbox to .pst files, users can free storage space on their mailservers.To use the .pst files from another location the user needs to be able to access the files directly over a network from his mail client. While it is possible to open and use a .pst file from over a network, this is unsupported, and Microsoft advises against it, as .pst files are prone to corruption when used in this manner.

Both the .pst and .ost files use a fixed-block-based allocation scheme; the file is enlarged by a fixed amount of bytes, and the file internally maintains information about the allocated and non-allocated blocks. So, when data files like email messages are added to a .pst file, its file size is automatically adjusted by the mail client (if necessary). When mail is deleted from a .pst file, the size of the .pst file will stay the same, marking the space as unallocated so that it will hold future data items. Recently removed data items can actually be recovered from .pst and .ost files.

To reduce the size of .pst files, the user needs to compact them.

Data access

Password protection can be used to protect the content of the .pst files.However, Microsoft admits that the password adds very little protection, due to the existence of commonly available tools which can remove or simply bypass the password protection.The password to access the table is stored without the first and last XOR CRC-32 integer representation of itself in the .pst file. Outlook checks to make sure that it matches the user-specified password and refuses to operate if there is no match. The data is readable by the libpst project code.

Microsoft (MS) offers three values for the encryption setting: nonecompressible, and high.

  • None the .pst data is stored as plain text.
  • Compressible the .pst data is encrypted with a byte-substitution cipher with a fixed substitution table.
  • High (sometimes called “better”) encryption is similar to a WWII German Enigma cipher with three fixed rotors.

Note that neither of the two encryption modes uses the user-specified password as any part of the key for the encryption.


The .pst file format is supported by several Microsoft client applications, including Microsoft Exchange ClientWindows Messaging, and Microsoft Outlook. The .pst file format is an open format for which Microsoft provides free specifications and irrevocable free patent licensing through the Open Specification Promise 

The libpst project includes tools to convert .pst files into open formats such as mbox and LDAP Data Interchange Format. libpst is licensed under the GPL and is now included in Fedora 10.MVCOM is a commercially licensed COM Component that provides access to .pst files without MAPI.

There are tools to convert .pst to other formats or to upload to other online e-mails like Gmail, for example.

Sherlock Data Recovery Services Data Recovery facilities technicians are intimately familiar with this and can help you recover your Outlook PST Files.