How to Fix PST Is Not an Outlook Data File

You double-click an old archive and Outlook answers "File is not an Outlook data file (.pst)". Years of mail sit behind that message and it feels gone. In most cases your mail is still inside your file. One flag, one wrong folder or one broken header triggers it and five fixes below match eight causes.

Why does Outlook say PST is not an Outlook data file?

Outlook shows that message when it cannot open your file as valid PST. PST files store mail, contacts, calendars and tasks in strict structure. One damaged block or wrong attribute stops Outlook from mounting it. Eight causes explain almost every PST error:

• Corruption from power cuts, forced shutdowns, bad disk sectors or viruses.
• Read-only or hidden flag on files copied from a CD, backup drive or locked folder.
• A network share or USB drive. Outlook does not support live PST files there.
• Wrong extension. Renamed OST files, .pst.txt names and files from other applications are not PST files.
• An empty or truncated file, like 0 KB copies from failed backups.
• A size above its limit, which makes PST files unstable.
• An outdated Outlook build, bad add-in or broken Outlook profile.
• A PST still open in another window or background process.

Real user reports match these causes. One TechSpot user restored a PST backup from a reimaged PC, got that message in Outlook 2010 and 2013 and saw other PST files from that folder import fine. One Slipstick forum user lost access to a 7 GB archive after a ransomware infection and one reply asked whether a virus changed the file extension. One Microsoft Q&A thread on corrupt PST files lists sudden shutdowns, oversized files, drive errors, virus attacks and network drops.

How to fix a PST error with quick checks?

Start with five checks that take under two minutes. They fix many PST errors with no repair tool.

1. Enable file name extensions in File Explorer, then confirm your file ends in .pst, not .pst.txt or .ost.
2. Check file size. A 0 KB or tiny file contains nothing so restore from a backup.
3. Close Outlook fully. End any OUTLOOK.EXE process in Task Manager.
4. Right click your PST, select Properties, clear Read-only and Hidden, then select Apply.
5. Copy it to a local folder like Documents. Open it with File, Open & Export, Open Outlook Data File.

OST files renamed to PST always fail. OST and PST are different formats so renaming does not convert them. Open OST files through their own Outlook profile or export data first.

How to repair a PST file with ScanPST.exe?

Use ScanPST.exe, Microsoft's Inbox Repair Tool, which installs with classic Outlook. It fixes minor structural errors and saves a backup copy before repair. Many guides send you to wrong folders so match your install type to find it:

Microsoft's repair page lists (x86) root\Office16 for Outlook 2016 and 2019. Users on Microsoft Q&A report finding no ScanPST.exe there on Microsoft 365 and finding it under C:\Program Files instead. When neither folder has it, search drive C: for scanpst.exe.

1. Close Outlook and open your ScanPST.exe folder.
2. Open SCANPST.EXE, select Browse and choose your .pst file.
3. Select Start. Wait until scanning ends.
4. If errors appear, select Repair with the backup box ticked.
5. Start Outlook and open your repaired file.

Repeat ScanPST.exe if errors remain. That sequence fixes many PST errors with no extra software. Microsoft says some files need several passes and items deleted for good or damaged past repair do not return. Check your Lost and Found folder after repair. One Microsoft Q&A user reported missing subfolders after repair so do not delete your backup copy until your mail checks out.

What to do when ScanPST.exe does not recognize your PST?

ScanPST.exe says it does not recognize your file when its header is broken or your file is not PST. Test your header bytes first. Real PST files start with characters !BDN, which is hex 21 42 44 4E. Enter that command in PowerShell with your own path:

Format-Hex -Path "C:\Users\you\Documents\old.pst" -Count 4

You see 21 42 44 4E when your header is intact. Any other bytes mean damage at file start or another file type under PST name. Then try these steps:

• Restore an older copy from backup, File History or cloud storage.
• Check whether your file is really OST data. In one forum thread from 2007, a user whose PST failed in ScanPST renamed it to .ost, repaired it with ScanOST, renamed it back and recovered mail. Try it on copies only.
• Use a PST repair tool, which rebuilds files ScanPST.exe rejects. Our guide to Repair PST file without ScanPST tool includes that path.

What to do about an oversized PST file?

PST files past their limit become unstable and trigger errors. Limits depend on file type and Outlook version:

Microsoft sets ANSI files near 1.93 GB and caps them at 2 GB whatever registry values say. Past a warning level, Outlook blocks sending, copying and moving new items. Split old folders into a second PST file or archive them to fix an oversized PST. Our guide to Reduce Outlook mailbox size shows how and Outlook mailbox is full includes warning messages.

How to fix an Outlook data file error when nothing works?

Work down that list when quick checks and ScanPST.exe fail. Every step targets one cause.

1. Check your disk. Failing drives corrupt files again. Open Command Prompt as administrator and enter chkdsk C: /f /r for drives that store your PST. Repair your PST again once it finishes.
2. Start Outlook in Safe Mode. Start it with outlook.exe /safe. If it disappears, disable add-ins in File, Options, Add-ins, COM Add-ins, Go.
3. Update Outlook. Select File, Office Account, Update Options, Update Now.
4. Repair Office. Open Settings, Apps, select Microsoft 365 or Office, then Modify and Quick Repair.
5. Create a new profile. Open Control Panel, Mail, Show Profiles, Add, set it as default and attach your PST. One university service desk lists that reset for staff who hit it at first launch. Old profiles remain so you lose nothing.
6. Check antivirus. Pause live scanning for one test, because security software can lock PST files while it scans.

If Outlook reports disabled data file usage, see Outlook data file usage is disabled error. If Import and Export option is missing, see Import Export option greyed out in Outlook.

Does new Outlook open PST files?

New Outlook for Windows does not mount live PST files and it has no ScanPST.exe. Microsoft added PST import to new Outlook in March 2026 so you can bring mail from PST files into your mailbox. Microsoft says more PST features are coming. See Microsoft 365 Message Center post RM485739 for rollout notes.

Repair steps here apply to classic Outlook. If you see it in new Outlook, switch to classic Outlook or open your PST in tools that open PST files directly.

Which fix matches your PST error?

Match what you see to your fix, then start at that row.

How to stop PST errors from coming back?

• Prevent PST errors by keeping every live PST on a local drive and out of OneDrive, network shares and USB sticks.
• Close Outlook before you copy, move or back up PST files.
• Save dated backup copies of every PST you cannot lose.
• Split any PST near 40 GB into yearly files.
• Check your disk with chkdsk when Outlook closes unexpectedly twice, before errors repeat.
• Update Outlook so every build opens all PST versions.

Questions About PST Errors in Outlook

More PST and Outlook guides

Jared Young

Jared is an engineering graduate in Computer Science who specializes in data recovery and backup. He writes Corbett Software guides on email attachments, backups and mail migration.

Last checked 10 Oct 2026 against Microsoft docs and live steps.