Introduction to Outlook data files

When you use Microsoft Outlook, you need a place to keep your e-mail messages, calendar, tasks, and other items. This storage place, known as a data file, allows you to keep your data on your computer.

In this article

About Outlook data files

Determine your account types

Personal Folders files (.pst)

Offline Folder files (.ost)

Data file locations

Data file security

About Outlook data files

When Outlook saves items to your computer, it uses a type of data file called an Outlook Personal Folders file (.pst). If you are using a Microsoft Exchange Server account, your items are usually delivered to and saved on the mail server. To allow you to work with your messages even when you cannot connect to the mail server, Outlook offers Offline Folders, which are saved in another type of data file called an Offline Folder file (.ost) on your computer.

The primary distinctions between the two types of Outlook data files are:

  • The Outlook .ost files are used only when you have an Exchange Server account and choose to work offline or use Cached Exchange Mode.

  • The Outlook .pst files are used for POP3, IMAP, and HTTP accounts. When you want to create archives or back up your Outlook folders and items on your computer, including Exchange Server accounts, you must create and use additional .pst files.

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Determine your account types

The Account Settings dialog box displays the account type for all e-mail accounts in your Outlook profile.

  • On the Tools menu, click Account Settings.

    Screen image

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Personal Folders files (.pst)

A Personal Folders file (.pst) is an Outlook data file that stores your messages and other items on your computer. Personal Folders files are the most common format in which information in Outlook is saved by home users or in small organizations. Home users usually use an Internet service provider (ISP) to connect to the Internet. The ISP also provides one or more e-mail accounts. The most common types of accounts are referred to by their Internet protocol names — POP3 and IMAP or just POP and IMAP. Another type of account is an HTTP or Web-based account that works similar to IMAP e-mail accounts. All three account types use a .pst file.

Your items can also be moved or archived to a Personal Folders file (.pst). Because a .pst file is kept on your computer, it is not subject to mailbox size limits on the mail server. By moving items to a .pst file on your computer, you can free up storage space in the mailbox on your mail server. Outlook can be configured to deliver new items to a .pst file, but doing so has several disadvantages, including not being able to work with your items when you are using Microsoft Outlook Web Access with your Exchange Server e-mail account or when you are working on another computer.

Beginning with Microsoft Office Outlook 2003, a newer .pst file was introduced that offers greater storage capacity for items and folders and supports multilingual Unicode data. A file that is created in the Outlook Personal Folders file (.pst) format in Outlook 2003 or Microsoft Office Outlook 2007 is not compatible with earlier versions of Microsoft Outlook and cannot be opened by using those versions. To create a data file that is compatible with Outlook 2002 and earlier versions, you can create data files in the Outlook 97-2002 Personal Folders file (.pst) format. This file format is the same as the format that was available in earlier versions of Outlook.

Tip: We recommend that you regularly back up your .pst files and store them in a safe place. Your ISP or Microsoft cannot recover your e-mail or other items if the .pst file is lost.

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Offline Folder files (.ost)

Typically when you use Microsoft Exchange Server, your e-mail messages, calendar, and other items are delivered to and stored on the server. You can configure Outlook to keep a local copy of your items on your computer in an Outlook data file called an Offline Folder file (.ost). This allows you to use Cached Exchange Mode or to work offline when a connection to the Exchange Server computer may not be possible or wanted. The .ost file is synchronized with the Exchange Server computer when a connection is available.

Offline folders are replicas of the folders found in your mailbox on the Exchange Server computer. They make it possible to take a folder from a server location, work with the contents of the folder when you are not connected to the network, and then, when you are connected again, update the folder and its corresponding server folder to make the contents of both folders identical. This process is called synchronizing folders.

You can add, delete, and modify the contents of an offline folder exactly as you can for a folder on a server. For example, you can change and move items between folders, send messages that are placed in your offline Outbox, and view the contents of your offline public folders. Meanwhile, new messages are kept in your Inbox on the server, and other people might add, delete, and change items in public folders. You will not be aware of these changes on the server until you synchronize.

The information that is synchronized includes:

  • Headers    For e-mail items only, a header is a descriptive identifier that provides the sender's name, the subject line of the message, the time when the message was received, and the size of the message.

  • Full items    A full item includes the header, the body of the message, and any attachments, including embedded objects or pictures.

When you work offline, folders that are synchronized are determined by Send/Receive groups. With Send/Receive groups, you can choose which folders are synchronized and kept current so that when a connection to the server is not possible or you choose to work offline, you can continue to work with those items. You can also specify that updates to the Address Book be downloaded during synchronization.

If you use an Exchange Server e-mail account, we recommend that you use Cached Exchange Mode. Most of the reasons to work offline are eliminated when you use Cached Exchange Mode. The lack of a network connection is virtually transparent to you because you can continue to work with your items.

By default, Cached Exchange Mode creates and uses an Offline Folder file (.ost) and then downloads and maintains a synchronized copy of the items in all folders in your mailbox. You work with the information on your computer, and Outlook synchronizes the information with the server.

Whether you are at the office, at home, or on an airplane, network changes and availability are transparent to you. When your connection to the Exchange Server computer is interrupted, you can continue to work with your data. When a connection is restored, Outlook automatically synchronizes changes, and the folders and items on the server and on your computer are identical once again. Outlook manages your connection to the server and keeps your data up-to-date. There is no need to switch to working offline and no need to keep trying to reconnect to the server — it is all automatic.

Cached Exchange Mode also frees you from having to set up Send/Receive groups, choose folders that you want to be available offline, and keep those folders synchronized. All those tasks are handled by Outlook.

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Data file locations

The following is a list of where Outlook data files are saved on your computer. Some of the folders might be hidden. To view them, do one of the following:

  • Microsoft Windows XP and Microsoft Windows Server 2003    

    1. Click Start, and then click My Computer.

    2. On the Tools menu, click Folder Options, click the View tab, and then click the Show hidden files and folders option.

  • Microsoft Windows 2000    

    1. Double-click My Computer on your desktop.

    2. On the Tools menu, click Folder Options, click the View tab, and then click the Show hidden files and folders option.

Outlook data files (.pst)    drive:\Documents and Settings\user\Local Settings\Application Data\Microsoft\Outlook

Offline Folders file (.ost)     drive:\Documents and Settings\user\Local Settings\Application Data\Microsoft\Outlook

To see where additional configuration files are saved, such as menu bar customizations, see Outlook File Locations in the See Also section. Keep these points in mind:

  • You can save, copy, and move a data file (other than the file that is used as your default delivery location) to another location on your hard disk or to a share on the network. However, you must have folder read/write permissions to open a Personal Folders file (.pst).

  • A data file can be accessed by only one user or program at a time.

  • The .pst data file can be saved to read-only media such as CDs and DVDs. However, to use the .pst file, you must copy the file to your hard disk drive.

Warning: We do not recommend accessing a data file from a network share or another computer, because it increases the possibility of data loss.

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Data file security

When you create a .pst file, you can assign a password to the file. A password improves security and reduces the ability of others to view your data. You can select the Save this password in your password list check box to avoid being prompted for the password each time you open the file.

Use strong passwords that combine uppercase and lowercase letters, numbers, and symbols. Weak passwords don't mix these elements. Strong password: Y6dh!et5. Weak password: House27. Passwords should be 8 or more characters in length. A pass phrase that uses 14 or more characters is better.

It is critical that you remember your password. If you forget your password, Microsoft cannot retrieve it. Store the passwords that you write down in a secure place away from the information that they help protect.

Note: If your Windows user account is not password-protected, or other people have access to your Windows user account, do not use the Save this password in your password list option.

create microsoft personal folders dialog box

1. By default, if you do not type a name in the Name text box, the Personal Folders file (.pst) is named Personal Folders in the Outlook Navigation Pane. This might be confusing if you open several .pst files, all with the same name. We recommend that you type a name that is meaningful to you.

2. A password is not required for a .pst file but helps to provide better protection for your information. Type the same password into both the Password and Verify Password text boxes. For security, dots appear as you type the password.

3. If you use a password for this .pst file, you can select this check box to prevent you from being required to type the password each time you open the .pst file in Outlook. Even if the .pst file is moved to another Windows profile or another computer, a password prompt still appears.

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