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Introduction to Outlook Data Files (.pst and .ost)

When you use Microsoft Outlook, your email messages, calendar, tasks, and other items are saved on a mail server, on your computer, or both. Outlook items that are saved on your computer, are kept in an Outlook Data File (.pst and .ost).

About Outlook Data Files (.pst and .ost)

There are two types of Outlook Data Files used by Outlook. An Outlook Data File (.pst) is used for most accounts. If you are using a Microsoft Exchange account, your items are usually delivered to and saved on the mail server. To allow you to work with your messages even when you can’t connect to the mail server, a second type of data file that is named an offline Outlook Data File (.ost) is kept on your computer.

The primary differences between the two types of Outlook data files are as follows:

Outlook Data Files (.pst) are used for POP3, IMAP, and web-based mail accounts. When you want to create archives or back up your Outlook folders and items on your computer, such as Exchange accounts, you must create and use additional .pst files.

Outlook Data Files (.ost) are used when you have an Exchange account and want to work offline or use the default Cached Exchange Mode. This type of data file is also used for accounts that you set up with the Outlook Connector for Outlook Data Files (.ost) are always copies of items that are saved on a mail server and don’t have to be backed up like Outlook Data Files (.pst).

Outlook Data File (.pst)

A Personal Folders file (.pst) is an Outlook data file that stores your messages and other items on your computer. This is the most common file 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 email accounts. The most common types of accounts are referred to by their Internet protocol names — POP3 and IMAP. Another type of account is an HTTP or web-based account that works similar to IMAP email accounts. All three account types use a .pst file.

Your items can also be moved or archived to an Outlook Data 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 if you do this, it has several disadvantages. This includes being unable to work with your items when you are using Microsoft Outlook Web Access with the Exchange Server email account or when you are working on another computer.

WARNING:   Do not access an Outlook Data File (.pst) from a network share or another computer, because it increases the possibility of data loss.

WARNING:   Do not grow your PST past 5GBs.  Complications can occur and corruption of the PST can happen.  If the PST grows to a size that is too large to manage then it is possible to not be able to recover it using the Scan PST tool.

TIP:   You should regularly back up your Outlook Data Files (.pst) and save them in a safe place. Your ISP or Microsoft can’t recover your e-mail or other items if the file is lost.

Offline Outlook Data File (.ost)

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

Offline folders are replicas of the folders found in your mailbox on the computer that is running Microsoft Exchange. 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 known as synchronizing folders.

You can add, delete, and change 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 included 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’ll not be aware of these changes on the server until you synchronize.

The information that is synchronized includes the following:

  • Headers:    For email 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, such as embedded objects or pictures.

When you work offline, folders that are synchronized are determined by Send/Receive groups. By using 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 email 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 almost transparent to you because you can continue to work with your items whether you are connected to the computer that is running Exchange.

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. When your connection to the Exchange computer is interrupted, you can continue to work with your data. When a connection is restored, changes are automatically synchronized, and the folders and items on the server and on your computer are identical again.

With Cached Exchange Mode, you do not have to set up Send/Receive groups, choose folders that you want to be available offline, and then keep those folders synchronized.

File locations

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 computer or to a share on the network. However, you must have folder read/write permissions to open an Outlook Data File (.pst).

Outlook Data Files (.pst)

NOTE:   Microsoft Exchange Server accounts save your information on the mail server. To use Cached Exchange Mode or to work offline, copies of your items are saved in an offline Outlook Data File (.ost). See the Outlook Data Files (.ost) section for more information. Also, some organizations allow you to export or archive your items to a .pst file.

The fastest way to open the folder where your Outlook Data File (.pst and .ost) is saved is to do the following:

  1. In Outlook 2010, click the File tab.
  2. Click Account Settings, and then click Account Settings.
  3. On the Data Files tab, click an entry of the data file you want to find, and then click Open Folder Location.

Outlook Data Files (.pst) created by using Outlook 2010 are saved on your computer in the Documents\Outlook Files folder.

If you upgraded to Outlook 2010 on a computer that already had data files that were created in earlier versions of Outlook, these files are saved in a different location in a hidden folder.

Windows 7 and Windows Vista    C:\Users\user\AppData\Local\Microsoft\Outlook

Outlook Data File (.ost)

The .ost file is synchronized with the items on the server that runs Exchange. Because your data remains on the Exchange server, you can re-create this .ost file on your new computer without having to back up the .ost file.

Windows 7:  C:\Users\user\AppData\Local\Microsoft\Outlook

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