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

When you use Microsoft Outlook, e-mail messages, calendars, tasks, and other items are saved on the mail server or computer, or they are saved in both locations. If Outlook items are saved on your computer, they are saved in Outlook data files (. pst and. ost).

About Outlook data files (. pst and. ost)

Outlook uses two types of Outlook data files. Outlook data files (. pst) are used for most accounts. If you are using a Microsoft Exchange account, the project is typically routed and saved to the mail server. To enable you to use messages when you cannot connect to the mail server, Outlook also saves the second type of data file that is called an offline Outlook data file (. ost) on your computer.

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

• Outlook data files (. pst) are used for POP3, Internet Message Access Protocol (IMAP), and Web-based mail accounts. If you want to create an archive or backup of Outlook folders and items (including Exchange accounts) on your computer, you must create and use an attached. pst file.

• If you have an Exchange account and want to work offline or use the default cached Exchange mode, you need to use an Outlook data file (. ost). This type of data file is also used for accounts that you set up using Outlook Connector for Windows Live Hotmail. An Outlook data file (. ost) is a copy of a project that is always saved on the mail server and does not need to be backed up like an Outlook data file (. pst).

Outlook data file (. pst)

A Personal Folders file (. pst) is an Outlook data file that is used to store your messages and other items on your computer. It is the most common file used by home users or small organizations to hold information in Outlook. Home users typically connect to the Internet through their ISP. The ISP also provides one or more e-mail accounts. The most common types of accounts are referenced by their Internet protocol name (POP3 and IMAP). Another type of account is an HTTP or Web-based account that works like an IMAP e-mail account. These three account types use. pst files.

Items can also be moved or archived to an Outlook data file (. pst). Because the. pst file remains on your computer, it is not constrained by the mailbox size limit on the mail server. By moving items to a. pst file on your computer, you can free up storage space for the mailbox on the mail server. Outlook can be configured to transfer new items to a. pst file, but there are several drawbacks to doing so, such as when you use Microsoft Outlook Web Access with your Exchange e-mail account or when you work on another computer, your project will not be available.

warning do not access Outlook data files (. pst) from a network share or another computer because this increases the likelihood of data loss.

Tip You should back up your Outlook data files (. pst) regularly and keep them in a safe place. If the file is missing, your ISP or Microsoft cannot recover your e-mail or other items.

Offline Outlook data files (. ost)

When you use Microsoft Exchange, e-mail messages, calendars, and other items are typically delivered and stored on the server. You can configure Outlook to save a local copy of an item on your computer in an Outlook data file called an offline Outlook data file (. ost). This allows you to use cached Exchange mode or work offline when you cannot connect or you do not want to connect to an Exchange computer. When you can make a connection, the. ost file is synchronized with the Exchange computer.

An offline folder is a copy of a folder in a mailbox on an Exchange computer. With these replicas, you can detach the folder from the server location, use the contents of the folder without connecting to the network, and then update the folder and its corresponding server folder after the connection is re-established so that the two folders have the same content. This process is called "Synchronizing folders."

You can add, delete, and modify the contents of an offline folder just as you do for a folder on a server. For example, you can change folders or move items between folders, send messages that are placed in the offline Outbox , and view the contents of offline public folders. At the same time, new messages remain in the Inbox on the server, and others can add, delete, and change items in the public folder. You will not be aware of these changes on the server until you synchronize.

The information that is synchronized includes:

• the header message header is for an e-mail item only, and is a descriptive identifier that provides information such as the sender's name, the subject line of the message, the time the message was received, and the message size.

• Complete Project A complete project contains headers, message bodies, and any attachments, including embedded objects or pictures.

When you work offline, the folders that are synchronized are determined by the send and receive groups. With Send/Receive groups, you can choose which folders to sync and keep up-to-date so that you can continue to use those items when you cannot establish a server connection or choose to work offline. In addition, you can specify that updates to the Address Book are downloaded during synchronization.

If you use an Exchange e-mail account, we recommend that you use Cached Exchange Mode. If you use cached Exchange mode, most of the reasons for working offline do not exist. The lack of a network connection does not actually have any effect on you, because you can continue to work on the project regardless of whether you are connected to the server 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 of the mailbox. You are dealing with information on your own computer, and Outlook is responsible for synchronizing that information with the server. You can continue to use your data when your connection to the Exchange computer is interrupted. When the connection is restored, the changes are automatically synchronized, and the server and the folders and items on your computer are exactly the same.

Under cached Exchange mode, you do not have to set up send and receive groups, you can choose which folders to use when you are offline, and synchronize those folders.

how to reduce. pst Footprint

Do you find that when you delete a large amount of email, you find that the PST file occupies no less capacity at all. So how can you really reduce the space occupied by PST files?

Very simple
When you delete a message in Outlook, you use "Delete all", or you don't actually delete the message, just mark the place where the message should be in a PST file. The "Compress" feature of PST frees the space occupied by these deleted messages, thereby reducing the size of the PST.

Reduce PST file space usage:
1. Delete messages
2. Completely delete messages from deleted messages
3. On the File menu, click Data File Management.
4. Click the data file that you want to compress, and then click Settings.
5. Click "Start Compression".

Note: After you compress a. pst file, you do not have to quit Outlook

Article derived from: https://support.office.com/zh-cn/article/Outlook-%E6%95%B0%E6%8D%AE%E6%96%87%E4%BB%B6%EF%BC%88pst-%E5%92% 8c-ost%ef%bc%89%e7%ae%80%e4%bb%8b-6d4197ec-1304-4b81-a17d-66d4eef30b78?ui=zh-cn&rs=zh-cn&ad=cn

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