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When you use Microsoft Outlook on a Windows computer, 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)
Outlook Data Files (.pst), or Personal Storage Tables files, contain Outlook user messages and other Outlook items, such as contacts, appointments, tasks, notes, and journal entries. You might also use an Outlook data file to backup messages or store older items locally on your computer to keep the size of your mailbox small.
Microsoft currently provides different levels of support for Outlook Data Files, depending on which version of Outlook you are using. To determine which version of Outlook you're using, look for the word Outlook on the top app bar. If you only see the Outlook icon, you're using classic Outlook. If the product name is next to the Outlook icon, you're using new Outlook.
Outlook Data File (.pst)
A Personal Folders file (.pst) is an Outlook data file that stores your messages and other items on your computer. 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 accessing it through the web browser or when you are working on another computer.
The Outlook PST File is located in one of the following two locations, which depend on your Windows version:
- Window 10 and 11 – C:\Users\%username%\Documents\Outlook Files
- Older Windows versions – C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook
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.
Default OST File Location in Windows 10/11:
C:\Users\%username%\AppData\Local\Microsoft\Outlook\
or
%LOCALAPPDATA%\Microsoft\Outlook\
Note: Replace your username with your actual Windows user profile name.
Additional information on the location of your Outlook Data Files can be found in the "Find and transfer Outlook data files from one computer to another" article.