About MS Outlook PST and OST data file

What Is MS Outlook?

MS Outlook or Microsoft Outlook is a personal information manager that comes as a part of Microsoft Office Suite. Even though being popularly used as an email client, it also comes with other very helpful features such as the calendar, contact manager, task manager, notes and web browser. Although being a part of the MS Office suite it can be downloaded separately. It can be used as a standalone application as well as with MS exchange Server and MS SharePoint Server for multiple users in an organization, a few common examples of their integration are: shared mailboxes, calendars, Exchange public folders, meeting schedules and SharePoint lists.

As we start using MS Outlook, all our emails, tasks, calendar and other such items are saved on either a mail server or on our computers, or both. The MS Outlook data items saved in our computer are stored in MS Outlook data files, that are, Personal Storage Table (.pst) and Offline Storage table (.ost).

What are the Data files in MS Outlook?

Before moving further with the data files we should first get familiar with following encodings:

1. ANSI data files
2. Unicode data files

ANSI data file format

An ANSI data file (.pst or .ost) format is a standard maintained for data file that is given as per the American National Standard Institute, which has a file size limitation of 2 GB (2³¹bytes). If the size of the file exceeds this limit the data might get corrupted. Moreover this is an 8 bit character encoding format hence, causing the system to slow down. This is why the development of the Unicode encoding came into picture.

Unicode data file format

Unicode data file (.pst or .ost) format can be defined as the improvised version of the ANSI format. It has overcome one of the biggest problem i.e. file size limitation to 2 GB in ANSI format. Moreover it is 32 bit character encoding format, because of which it support multiple character set and has no limit for the no of items per folder hence the user is free to save his data without any worrying about the data size.

Now as we are familiar with the data encodings we can very easily understand the two file extensions in MS Outlook:-

• PST files –  Personal Storage Table data files
• OST files – Offline Storage Table data files

The Personal Storage Table (.pst) data files are used as the data file in MS Outlook for accounts that are synchronized with Outlook where as Offline Storage table (.ost) is created when we activate the cached mode while configuring a new mailbox while we are using it with Microsoft Exchange Server.

A further description of the data files is elaborated in the Q/Ans. Below:

Q:          What are PST Data files in MS Outlook?

A:         PST is an abbreviation that is used for Personal Storage Table. In MS Outlook the messages,calendar and other data items are saved in PST files in short we can say that a PST data file is store room for all our outlook data for a particular account. The PST file is usually used to store archived data. The file is enlarged by a particular amount of bytes that is done by the use of a fixed block based allocation scheme. This file then internally keeps a record of the information about the non allocated and allocated blocks. So, whenever any data file like email message or any noteis added to a PST file, the mail client automatically adjusts the file size of the PST file.

2 GB File size limit

             Until the introduction of MS Outlook 2003 the PST files were having a limitation of 2 GB i.e. as soon as the size of a PST file reaches close to 2 GB the Outlook starts to slow down and as the size reaches 2 GB the data files got corrupted. The main reason for this was till 2002 version of MS Outlook,it used the ANSI character encoding that i.e. American National Standard Institute that was limited up to only 2 GB data storage capacity. This was one of the major drawbacks of MS Outlook.

Seeing this as a major problem, Microsoft gave a solution to this problem in the form of a compression tool: Exchange 2000 tool - PST 2GB. But this tool also had a drawback. It compressed the PST file by truncating the data and creating a copy of the original PST data file. This truncated copy did not have all the data resulting in loss of data. Hence this tool didn’t prove to be a complete solution to the problem.

So in order recover the lost outlook email data the users had to either get a technician or use a third party tool. The lost data can be Email, calendar, journal, task list and so on….  

Q:          Where are the PST data files in MS Outlook?

A:         The location of the PST data files may vary according to the windows version that we are using. So accordingly the following paths are defined as per the windows version:

• For MS Outlook 2013

• Windows 8:

• Win 7/vista:

• Windows XP:

• For Other versions

• Win 7/8/vista:

• Windows XP:

Q:          What are OST Data files in MS Outlook?

A:       OST is an abbreviation used for Offline Storage table data file. These files are created when our MS Outlook is configured with an Exchange Server Account and while configuring a new mail account we have used the cached mode option. When we use Outlook with Exchange server all our emails, journals and other items are delivered and stored on the server. The Outlook data file (.ost) keeps a copy of these data on our local machine. The cached exchanged mode allows us to work offline and whenever there is no connection with the exchange machine. The Outlook data file (.ost)is synchronized with the exchange machine as soon as a connection is established.

Q:        Where are the  OST data files in Outlook?

A:       The location of the OST data files may vary according to the windows version that we are using. So accordingly the following paths are defined as per the windows version:

• For windows 8/7/vista

• For windows XP