The message “The file … is not an Outlook data file (.pst)” means Outlook could not read the file as a PST: it may be damaged, it may not be a PST at all, or the copy Outlook is pointing at may be incomplete. Work on a copy, check the first bytes of the file to see whether it really is a PST, and run Microsoft’s Inbox Repair Tool if it is damaged. If the header shows a readable Unicode PST and you only need the mail out of it, PstVault, a Windows app from Automata Labs, opens the PST read-only without Outlook and exports the messages you choose.
This guide covers what the error usually means, a 30-second header check you can run in PowerShell, Microsoft’s repair route, and when a viewer is the faster way to the mail.
Open a Unicode PST without Outlook and export the messages you need, read-only.
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What does “is not an Outlook data file (.pst)” mean?
Outlook shows it when it opens a file listed as a data file and the contents do not look like a usable PST. The full message often reads:
Cannot start Microsoft Outlook. Cannot open the Outlook window.
The set of folders cannot be opened. The file C:\...\archive.pst
is not an Outlook data file (.pst).The same wording appears for OST files, ending in “(ost)” instead, when an offline cache is the problem. Microsoft’s Outlook and OWA synchronization troubleshooting article quotes that OST version of the message.
The causes fall into three groups:
| Cause | What you will see | First step |
|---|---|---|
| The PST is damaged | The header looks right, but Outlook still refuses it | Run the Inbox Repair Tool on a copy |
| The file is not a PST | The header check fails | Find the real archive or a backup |
| The PST lives in a synced or remote folder | The file is in OneDrive or on a network share | Move a copy to a local folder first |
How do you check whether a file is really a PST?
Every PST starts with a fixed header. Microsoft’s file format specification, MS-PST HEADER, says the first four bytes must be ”!BDN”, the client signature at offset 8 must be the two bytes 0x53 0x4D (“SM”), and the version field at offset 10 tells the format apart: it must be 14 or 15 for an ANSI PST, and 23 or greater for a Unicode PST.
That means you can tell in seconds whether the file is a PST, and which kind, without Outlook. Save this as pst-header.ps1 and run it with the path to a copy of your file:
param($Path)
$bytes = New-Object byte[] 12
$fs = [System.IO.File]::OpenRead($Path)
try { [void]$fs.Read($bytes, 0, 12) } finally { $fs.Close() }
$sig = [System.Text.Encoding]::ASCII.GetString($bytes, 0, 4)
$client = [System.Text.Encoding]::ASCII.GetString($bytes, 8, 2)
$ver = [BitConverter]::ToUInt16($bytes, 10)
"Signature: $sig Client: $client wVer: $ver"
if ($sig -ne '!BDN' -or $client -ne 'SM') { 'Not a PST header' }
elseif ($ver -eq 14 -or $ver -eq 15) { 'ANSI PST (older format)' }
elseif ($ver -ge 23) { 'Unicode PST' }
else { 'Unrecognized version' }powershell -ExecutionPolicy Bypass -File .\pst-header.ps1 -Path "D:\Recovered\archive-copy.pst"The script only reads the first 12 bytes and never writes to the file. We tested it on sample headers built from the specification:
| Result | What it means | What to do next |
|---|---|---|
| Signature !BDN, Client SM, wVer 23 or more | A Unicode PST header | Repair it if Outlook still refuses it, or open it with a viewer |
| Signature !BDN, Client SM, wVer 14 or 15 | An ANSI PST from Outlook 97 to 2002 | Open it in Outlook or a tool that supports ANSI files |
| Anything else | Not a PST as the spec defines it | Find the real archive; renaming a file to .pst does not convert it |
A header that passes does not prove the rest of the file is healthy. It does tell you that the file is the right kind of thing, which rules out the most common wrong turn: spending an afternoon repairing a file that was never a PST. Our explainer on what a PST file is covers the ANSI and Unicode formats in more depth.
How do you repair a PST with the Inbox Repair Tool?
If the header is right and Outlook still will not open the file, Microsoft’s route is the Inbox Repair Tool, SCANPST.EXE. Microsoft’s repair article lists “You receive a message that Outlook can’t open your data file” as one of the situations it is for, and recommends backing up the data file before you start. The steps, in short:
- Exit Outlook.
- Find SCANPST.EXE in your Office folder. Microsoft gives
C:\Program Files (x86)\Microsoft Office\root\Office16for Outlook 2016 and 2019. - Browse to the PST, choose Start, and choose Repair if errors are found.
SCANPST comes with desktop Outlook, so it is not an option on a PC that does not have classic Outlook installed.
Why does OneDrive matter for this error?
A PST that Outlook keeps open is written to constantly, and sync tools copy files while they change. Microsoft’s article on how to remove an Outlook .pst data file from OneDrive says a large data file in OneDrive can slow sync and show “Processing changes” or “A file is in use,” and walks through copying the PST to a folder outside OneDrive and pointing Outlook at the copy.
Before you repair anything, make a local copy outside OneDrive, a network share or a USB stick, and work on that.
Already have a readable PST and just need the mail? PstVault opens it without Outlook.
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When is it faster to open the PST without Outlook?
Often the goal is not to attach the PST to Outlook again. It is to find a conversation, save attachments, or hand a set of messages to someone. In that case, a viewer that opens the file read-only may get you there with less risk than repairing and reattaching it.
| Situation | Better route |
|---|---|
| You need the archive back inside Outlook as a working mailbox | Repair it with SCANPST, then attach it |
| You need specific messages or attachments out of a Unicode PST | Open it with a viewer such as PstVault |
| Outlook is not installed on this PC | A viewer; SCANPST ships with desktop Outlook |
| The header check failed | Neither; find the real archive or a backup |
| The file is an ANSI PST | Outlook or a tool that supports ANSI files |
What can PstVault do with a PST that Outlook refuses?
PstVault opens Outlook data files without Outlook. According to its Microsoft Store listing:
- Read-only. It opens your PST read-only and creates new output files without overwriting existing ones, so the original archive stays unchanged.
- Find messages. Browse the folder tree with message counts, and filter by sender, recipient, subject, words in the body or a local date range.
- Export what you need. Save one email or a batch as PDF (individual files or one bookmarked PDF), EML with available attachments, or offline HTML. Contacts go to CSV or vCard, and supported calendar events to ICS.
- Know what worked. An export report marks items as complete, partial, failed or not processed, with output SHA-256 hashes in the JSON report.
- Stays local. Mail processing happens on your PC; PstVault does not upload your email or connect to a mail server.
The limits matter for this error, so here they are plainly, from the same listing: PstVault is designed for readable Unicode PST files, one archive at a time. It does not open OST files, repair damaged archives or decrypt encrypted messages. If your header check says Unicode PST, the free trial is the quickest way to see whether PstVault can read it. If the file is badly damaged, repair comes first.
Our guides to opening a PST without Outlook and exporting emails from a PST file cover the export side step by step.
Frequently asked questions
Can I fix this error by renaming the file to .pst?
No. The extension does not change the contents. If the header check does not show ”!BDN” and “SM”, the file is not a PST.
Is an OST file the same as a PST?
No. An OST is an offline cache of a server mailbox. Microsoft’s repair article notes that for an Exchange account you can delete the OST and Outlook recreates it. PstVault does not open OST files.
Does SCANPST delete mail?
Microsoft recommends backing up the data file before you run it, and the scan creates a backup file. Run it on a copy if the archive matters.
Can PstVault open a PST that SCANPST cannot repair?
PstVault does not repair damaged archives. If the file is readable enough to open, it can export what it reads and reports partial or failed items. The free trial shows you before you buy.
Try PstVault
PstVault is a one-time purchase in the Microsoft Store with a free trial. There is no subscription and no account. More detail is on the PstVault product page.
Get the messages out of an old Unicode PST without Outlook, and keep the original untouched.
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Microsoft, Outlook and OneDrive are trademarks of the Microsoft group of companies. PstVault is not affiliated with Microsoft.
Last updated 21 October 2026. Published by Automata Labs, a software company in Canada.