Email Sync Problems

Does Microsoft 365 Outlook Use OST or PST Files on Windows 10?

Maira Mehtab Sep 22, 2026 870 views

Question details

The user wants to understand if Outlook in Microsoft 365 stores email data in an OST or PST file format on a Windows 10 computer.

Product: Microsoft 365 Outlook
Device & OS: Windows 10
Scenario: Checking local email data storage configuration to manage space or backup data.
Observed behavior: Outlook data files are determined by the specific email protocol used (POP, IMAP, Exchange) rather than the Microsoft 365 subscription itself.

Before you start

Before checking your data files, ensure your Outlook desktop app is open and verify which email protocol (POP3, IMAP, or Exchange) your accounts are currently using.

Solution 1Recommended

Check Data File Type in Outlook Account Settings

Determine whether your specific Microsoft 365 email configuration utilizes an OST or PST file by viewing the built-in Outlook Data Files tab.

Microsoft 365 itself is a subscription suite and does not dictate a single data file type. Instead, the type of file Outlook creates depends entirely on how your email account is configured.

If you configure an email account using POP, Outlook uses a Personal Storage Table (.pst) file. If you use an Exchange account or most modern IMAP accounts, Outlook creates an Offline Storage Table (.ost) file to synchronize a local copy of your emails.

Open Account Settings

Launch the Microsoft 365 Outlook desktop app. Click on 'File' in the top left corner, then select 'Info'. Click 'Account Settings' and choose 'Account Settings' from the drop-down menu.

View the Data Files Tab

In the Account Settings window, switch to the 'Data Files' tab.

Identify File Extensions

Look at the 'Location' column to see the file extension (.ost or .pst) associated with each of your email accounts.

Default File Locations: By default in Windows 10, PST files are generally stored in C:\Users\[Username]\Documents\Outlook Files, while OST files are stored in a hidden system folder located at C:\Users\[Username]\AppData\Local\Microsoft\Outlook.

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Frequently Asked Questions

Can I convert my OST file to a PST file?

Yes. You can export your OST data to a PST file by opening Outlook, going to File > Open & Export > Import/Export. Choose 'Export to a file', select 'Outlook Data File (.pst)', and follow the prompts to save your mailbox data.

What happens if I delete my OST file?

Because an OST file is simply a cached offline copy of your server mailbox, deleting it is generally safe. The next time you open Outlook and connect to the Exchange or IMAP server, Outlook will automatically recreate the OST file and download your emails again.

Why does my IMAP account use an OST file instead of a PST?

In Outlook 2013 and newer versions (including the current Microsoft 365 versions), IMAP accounts default to using OST files to store offline cached data. Older legacy versions of Outlook used PST files for IMAP accounts.

How do I move my PST file to another drive on Windows 10?

To move a PST file, first close Outlook completely. Cut the .pst file from its current folder and paste it into the new destination. When you reopen Outlook, it will prompt you that the data file is missing; simply browse to the new location, select the file, and click OK.

Outlook & Email Problems