Microsoft Outlook uses two types of data files: Outlook Data File (PSTs) and Offline Outlook Data Files (OSTs). A 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 cannot connect to the mail server, an OST file is kept on your computer.
The primary differences between the two types of Outlook data files are as follows:
- PST files 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.
- OST files are used when you have an Exchange account and want to work offline or use 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.com (formerly Hotmail). OST files are always copies of items that are saved on a mail server and do not have to be backed up like PST files.
For more information, refer to the Microsoft Office article Introduction to Outlook Data Files.
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