An introduction to Outlook Data Files (.pst and .ost)

A local copy of your information is stored on your computer when you add an email account to Outlook. By using this feature, you can access previously downloaded or synchronized email messages, calendar information, contacts, and tasks without an Internet connection.

POP accounts, for example, store their information in Outlook Data Files (.pst files). Offline Outlook Data Files (.ost files) are used by Outlook 365 accounts, Exchange accounts, IMAP accounts, and Outlook.com accounts.

Outlook Data File (.pst)

Outlook Data Files (.pst) contain your emails and other items in Outlook. A POP account downloads all the emails from your mail server to your computer, which is a common type of account when adding an email account from an ISP such as Xfinity, AT&T, or Cox.

"Tip: If your Outlook Data File (.pst) becomes large it can slow down your OneDrive sync and may show "Processing changes" or "A file is in use". Learn How to remove an Outlook .pst data file from OneDrive."

Note: In Outlook 2013 and earlier versions, IMAP accounts also used an Outlook Data File (.pst). In Outlook 2016 and Outlook for Microsoft 365, IMAP accounts use Offline Outlook Data Files (.ost).

The Outlook Data File (.pst) can also be used to backup or export items from any type of email account. If you would like more information on backing up your email messages, calendar, contacts, and tasks, see Export or backup email, contacts, and calendar to an Outlook .pst file. Messages, calendars, contacts, and tasks can also be imported from another Outlook Data File (.pst). View Import emails, contacts, and calendars from Outlook .pst files for more information.

Note: that Offline Outlook Data Files (.ost) cannot be imported.

Since these files are saved on your computer, they are not limited by mailbox size limits on a mail server. You can free up mailbox storage space on your mail server by moving items to an Outlook Data File (.pst) on your computer.

In Outlook Data Files (.pst), messages and other items are only available on the computer where the file is saved.

Offline Outlook Data File (.ost)

Other account types, such as IMAP accounts, Microsoft 365 accounts, Exchange accounts, and Outlook.com accounts, use the Offline Outlook Data File (.ost) file to store your mailbox information locally. Even if your connection to the mail server is interrupted, for example, if you're on an airplane without WiFi or if your internet connection is disconnected, you will still be able to access any emails, calendar data, contacts, and tasks you've previously downloaded. Messages can be read and composed, appointments can be created, and contacts can be added. Your email messages will not be sent and no new messages will be downloaded until your internet connection is restored.

When the connection is restored, changes are automatically synchronized, and the folders and items on your computer and the server are identical again.

Group Storage File (.nst) for Outlook

A new type of Outlook data file is created the first time you access a Group in Outlook. It contains your Groups conversations and other local Groups data, and has an extension of .nst. There is no need to back up or import this file when you set up Outlook on a new computer because it is synchronized between your computer and the email server. 

File locations

To open the folder where your Outlook Data Files (.pst and .ost) are located, follow these steps:

• In Outlook, click the File tab.
• Next, select Account Settings from the right pane.

• Then click the Open Folder Location button under the Data Files tab.

Typically, Outlook Data Files (.pst) created using Outlook 2013 or Outlook 2016 are saved in Documents/Outlook Files on your computer. If you upgraded to Outlook on a computer that already had data files created in Microsoft Office Outlook 2007 or earlier, the data files are saved in a hidden folder at drive:\Users\user\AppData\Local\Microsoft\Outlook.

"Tip: If your Outlook Data File (.pst) becomes large it can slow down your OneDrive sync and may show "Processing changes" or "A file is in use". Learn How to remove an Outlook .pst data file from OneDrive."

Outlook Data Files (.ost) are stored on drive:\Users\user\AppData\Local\Microsoft\Outlook. Most data remains on the server; however, any items that are saved locally should be backed up. For example, Calendar, Contacts, Tasks, and any folders marked Local only.

"Tip: For information about hidden folders in Windows, see Windows Help and Support."