An Outlook PST file, or Outlook Data File, holds your messages and other Outlook items such as contacts, appointments, tasks, notes and journal entries on your own computer. You can open one in classic Outlook or new Outlook, but new Outlook still needs classic Outlook installed to open a PST, and importing a PST into a mailbox only works in classic Outlook as of October 2026.
Here's the catch that catches people out. If you put a password on a PST and forget it, it's gone for good. Microsoft says plainly that it can't retrieve it and there's no password recovery option. The other surprise is that deleting mail doesn't always make the file smaller: you often have to compact it yourself.
PST or OST: which one you've got
Not every account uses a PST. If you use a POP account, everything, including folders, email, contacts, calendar and tasks, lives in a PST. If you use a Microsoft 365, Exchange, IMAP or Outlook.com account, Outlook keeps a copy of your mailbox in an Offline Outlook Data File, which ends in .ost. That matters, because some things you can do to a PST, such as setting a password, aren't possible on an OST at all. You can see which you've got in the Location column of the Data Files tab.
What new Outlook and classic Outlook can do with a PST
To tell which version you're using, look at the top app bar. If you only see the Outlook icon, it's classic Outlook. If the product name sits next to the icon, it's new Outlook. Classic Outlook also has a File tab on the ribbon.
| Task | New Outlook, as of October 2026 | Classic Outlook |
|---|---|---|
| Open, read and search a PST | Yes, with classic Outlook also installed | Yes |
| Move, copy and delete emails and folders | Yes, including drag and drop to and from your mailbox | Yes |
| Export mail, calendar, contacts and tasks to a PST | Yes, from Settings, Files, Export | Yes |
| Import a PST into a mailbox | Not yet | Yes |
| See calendar and contacts saved in a PST | Not yet | Yes |
| Set a PST password or compact the file | Steps are in classic Outlook | Yes, from File, Account Settings |
Microsoft says it plans to add bulk importing of mail, calendar and contacts from a PST into new Outlook, and to drop the requirement for classic Outlook to be installed. After that, it says, it has no further investments planned.
How to open a PST in new Outlook
- Go to Settings, then Files, then Outlook Data Files.
- Select Add file.
- Select the .pst file, then select Open.
To close it again, go to the same Outlook Data Files screen, select Manage on the PST and choose Close file. If none of this works, you may not be on new Outlook for Windows yet, so follow the classic steps instead.
New Outlook has a few requirements Microsoft lists for PSTs. Classic and new Outlook must be the same architecture, both 32-bit or both 64-bit, and if you need to switch you have to uninstall the old version first. The account must have a Microsoft 365 subscription. PSTs aren't supported on the ARM version of new Outlook, and new Outlook can't be in Safe or Recovery mode.
When a PST is the right tool
Microsoft's own examples are moving mail from a personal account to a business one, moving from an old address to a new one, moving to a new PC, moving from a PC to a Mac, and keeping periodic backups. For a move between accounts, you add both accounts to Outlook on your desktop, wait for all the mail to appear, export the old account to a PST, then import that PST into the new account. If you only want the emails out, my guide to exporting Outlook emails covers that route in more detail.
One thing Microsoft is honest about: there's no automated way to back up everything, old and new, on a schedule. Auto-archive only covers old items, and it moves them instead of copying them. If that's what you want, read up on archiving emails in Outlook so you know exactly where your mail ends up. For a backup, export to a PST and copy it somewhere safe, such as a USB drive, another hard drive or cloud storage.
Fixing PST import errors
| Problem | Fix Microsoft gives |
|---|---|
| An error saying you don't have permission to access the file | If it's on a CD-ROM, copy it to your computer, open Properties and clear Read Only. If you switched from a local account to a Microsoft account while the file was in use, give your Microsoft account Full control under Properties, Security, Edit, then restart Outlook |
| The source and destination folders cannot be the same | Run the import again and choose Import items into the current folder |
| The service provider doesn't support the operation | The mailbox needs setting up as IMAP, which is often the case with Outlook.com; remove the account and add it back as IMAP |
| Emails came in, but not contacts or calendar | They probably weren't exported. Export again with Include subfolders ticked |
| Old subfolders are missing | Same cause and same fix: export again with Include subfolders ticked |
For the IMAP fix, Microsoft gives the incoming server as outlook.office365.com on port 993 with SSL, and the outgoing server as smtp-mail.outlook.com on port 587 with TLS, with My outgoing server requires authentication switched on. Its tip is that the most common reason the connection fails is a typo in the email address or password, and Outlook won't tell you that's the cause. If the file itself seems damaged, look at repairing Outlook before you go further.
Shrinking a PST that won't get smaller
When you delete items from a PST or OST, Outlook compacts the file in the background while your computer is idle and Outlook is still running, but only once there's enough unused space in the file. To do it now:
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- Delete what you don't want and empty the Deleted Items folder.
- Select File, then Account Settings, then Account Settings.
- On the Data Files tab, choose the file and select Settings. For an Exchange or Outlook.com account, also select the Advanced tab, then Outlook Data File Settings.
- Select Compact Now, then OK.
It can take several minutes, and you don't have to close Outlook afterwards. To find what's eating the space, try File, Tools, Mailbox Cleanup, which can show folder sizes, empty Deleted Items and clear the Conflicts folder. Or type messagesize:>5 mb in the search box to list every message over 5 MB.
Putting a password on a PST
- Click File, then Account Settings, then Account Settings.
- On the Data Files tab, pick the PST and click Settings.
- Click Change Password. If you don't see it, you've picked an OST.
- Type the old password, or leave it blank if there isn't one, then the new password twice, and click OK.
The password can be up to 15 characters and it's case-sensitive, so an accidental Caps Lock is a common reason it suddenly stops working. Microsoft's advice for keeping other people out is a password-protected Windows user account for each person, which is a better lock than the PST password.
My verdict on PST files
I'd use a PST for one-off jobs: moving accounts, moving computers, or keeping a backup you copy somewhere safe. I'd skip the PST password unless you'll store it somewhere you can't lose, because there's no way back if you forget it. And if you're on new Outlook and need to import, switch to classic Outlook for that job instead of fighting it.
Frequently asked questions
Can I open a PST file in new Outlook?
Yes. Go to Settings, Files, Outlook Data Files and select Add file. Classic Outlook must also be installed, in the same 32-bit or 64-bit architecture.
Can I import a PST file in new Outlook?
Not as of October 2026. Importing a mailbox or calendar from a PST needs classic Outlook.
What if I forget my PST password?
Microsoft can't retrieve it and there's no password recovery option.
Why didn't my PST file get smaller after I deleted emails?
Outlook only compacts the file in the background when there's enough unused space. Use Compact Now in the data file settings to do it straight away.
Can I put a password on an OST file?
No. There's no way to set a password on an Offline Outlook Data File (.ost).
