What does an NST File Store?
An NST file is a data file that Outlook for Windows creates for Microsoft 365 Groups. The file contains a local, synchronized copy of the shared mailbox and calendar of every Group that your mailbox joined.
Office Watch reports that a mailbox gets one NST, and the file sits in the same folder as the OST file. Outlook labels both files Outlook Data File, which confuses many users.
| Detail | NST |
|---|---|
| Extension | .nst file |
| Other name | Group Storage Table |
| Holds | Group conversations and Group calendars |
| Default folder | C:\Users\username\AppData\Local\Microsoft\Outlook |
| Backup needed | No, because the server holds the data |
Where does Outlook Save the NST File?
- Open Outlook and right click your Microsoft 365 account.
- Select Data File Properties and then Advanced.
- Select Outlook Data File Settings.
- Read the path in the File box.
Open the AppData folder by hand if you prefer. Show hidden items in File Explorer first.
How do You View the Data?
Method #1: Use Outlook (Free)
Start Outlook with the profile that owns the file. Open the Group in the folder list to see its conversations and calendar. Outlook on the web shows the same Group too.
Method #2: Use an NST Viewer (Paid After Demo)
Outlook cannot open an NST from another profile or a deleted account. An NST viewer opens the file directly, shows Group mail and exports it to PST. Test the demo on a copy of the file first.
Method #3: Rebuild the File (Free)
Close Outlook and rename the NST data so it ends in .old. Start Outlook, and the app creates a new file and syncs your Groups again. Our OST file guide explains the same rebuild for the OST file.
Is It Safe to Delete It?
Yes, when your account is active. Outlook recreates the file from the online mailbox, as Office Watch notes. Rename the file first and delete it after you check your Groups.
Retain the file when the account is gone, because it can be your only copy. Our Outlook data file errors guide lists the messages that appear when an Outlook data file fails.
Frequently Asked Questions
What does the File Contain?
An NST is the local copy of the Microsoft 365 Group mailboxes that Outlook maintains for your account.
Where is the NST Data in Windows?
Outlook saves the file in AppData\Local\Microsoft\Outlook, next to the OST file.
Can You Delete It?
Yes, with an active account. Outlook recreates the file from the server. Rename it first.
Does It Need a Backup?
No. The file is a synchronized copy of online data, as Office Watch states.
Why does an NST Appear With No Groups?
Office Watch reports that Outlook can create a small NST when you set up a Microsoft 365 account, even before you use Groups.
Does a Double Click Open It?
No. Outlook controls the file. Use Outlook, Outlook on the web or an NST viewer.
Does IBM Notes Use NST Data Too?
Some sources state that the Outlook Connector for IBM Notes creates NST data too. Check your connector documentation.
Author's Verdict
An NST is a cache of your Groups, so rebuild it before you worry.
Use an NST viewer only when the account is gone and the file is your last copy.
Key Takeaways
- NST stores Microsoft 365 Group data locally.
- Outlook rebuilds the file from the server.
- Rename the file and do not delete it first.
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