When Outlook saves items to your computer, it uses a type of data file called an Outlook Personal Folders file (.pst) (Personal Folders file (.pst): Data file that stores your messages and other items on your computer. You can assign a .pst file to be the default delivery location for e-mail messages. You can use a .pst to organize and back up items for safekeeping.). 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) (Offline Folder file: The file on your hard disk that contains offline folders. The offline folder file has an .ost extension. You can create it automatically when you set up Outlook or when you first make a folder available offline.) 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 (POP3: A common protocol that is used to retrieve e-mail messages from an Internet e-mail server.), IMAP (IMAP (Internet Message Access Protocol): Unlike Internet e-mail protocols such as POP3, IMAP creates folders on a server to store/organize messages for retrieval by other computers. You can read message headers only and select which messages to download.), 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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OST Viewer Tool