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OST File Fix

Corrupt OST File Do Not Repair It Delete and Resync Instead

A corrupt PST is a data emergency. A corrupt OST is an errand. The difference is one fact - and it changes the entire fix.

By Technical AnshuOutlook 2010 to Microsoft 3657 minute read
The video covers repairing data files - this page explains the file you should not repair at all

ScanPST will happily accept an OST file, and many guides treat it exactly like a PST. That is wasted effort on a misunderstanding. An OST is an offline cache: for Exchange, Microsoft 365, and IMAP accounts, the real mailbox lives on the server, and the OST is merely a local copy Outlook keeps for speed and offline work. Repairing a cache is like repainting a mirror because the reflection looks wrong. The correct fix is to discard the cache and let Outlook download a fresh one - with one important stop-and-think exception, covered honestly below.

Know which file you are holding
account.ost - cached copy of the server mailboxDISPOSABLE
The mailbox itself - lives on the Exchange / Microsoft 365 / IMAP serverTHE ORIGINAL
archive.pst - a real data file, exists only hereREPAIR RULES APPLY
Same folder, same-looking icons, completely different rules. Check the extension before choosing a fix.
1

Why an OST Is Not a PST

A PST is a container: for POP accounts and archives, it may hold the only copy of years of mail. An OST is a reflection: delete it tonight and by morning Outlook has rebuilt it from the server, every folder intact. ScanPST passes on an OST spend hours lovingly repairing data that the server would replace for free - and can even preserve local glitches that a clean resync would have washed away.

Modern Outlook has quietly agreed with this logic for years: the separate OST integrity checker of old versions is gone, because the rebuild is the official repair.

2

The Rebuild, Step by Step

  1. Close Outlook completely
  2. Open the OST's folder - the path is shown in File, Account Settings, Data Files; the default location is C:\Users\YourName\AppData\Local\Microsoft\Outlook
  3. Rename the .ost to .old rather than deleting it outright - renaming is reversible if the exception below turns out to apply
  4. Start Outlook. It creates a fresh OST and resyncs the mailbox from the server
  5. Let it finish - the status bar shows the download progress; a multi-gigabyte mailbox on a slow connection can take hours, and Outlook stays usable meanwhile
  6. Once the resync is verified complete and Outlook is healthy for a few days, the .old file can be deleted
3

What a Resync Does Not Bring Back

Server data all returns. Local-only data does not, and this is the part the quick guides skip:

  1. Folders marked this computer only - anything stored in them exists solely in the OST
  2. Changes made while offline that had not yet uploaded - filed, flagged, or composed-but-unsent work from a flight or an outage
  3. In some IMAP setups, locally kept contacts and calendar items that never lived on the server

If any of that might exist, do not rebuild blind: open the renamed OST under a profile first, or export the suspect folders to a PST, then rebuild.

4

The Exception - Stop Before You Delete

The rebuild rests on one assumption: the mailbox still exists on the server and you can still sign in to it. Verify before touching anything when:

  1. The account was removed, the job ended, or the subscription lapsed - the server copy may be gone, making the OST the last copy in existence
  2. The account is an old POP-era setup - those mailboxes usually live in a PST (repair rules apply), but confirm which file you actually have
  3. Outlook has been used heavily offline recently

If the OST is the last copy, treat it like a PST: copy it, protect it, and use repair or extraction on the copy.

5

When OST Corruption Keeps Returning

Rebuilding monthly is not a fix; it is a symptom with a subscription. Recurring OST corruption points at a cause underneath: a failing disk (check its SMART health), an antivirus locking the file while Outlook writes, a mailbox grown enormous on a slow drive, or a damaged Outlook profile. Fix the cause - the rebuild should be a rare event in a healthy setup, not a routine.

The one-line test: ask "if this file vanished tonight, does the mail still exist somewhere?" For an OST on a live server account, the answer is yes - rebuild without fear. If the answer is no or "not sure", the file is data, not cache, and every PST protection rule in this series applies to it.

Watch the Complete Process in the Video

Watch the video for the repair craft - and remember that for an OST, the bravest and fastest move is the rebuild.

Watch Full Video
Common Questions

How long does the resync take?

It depends on mailbox size and connection speed - a few gigabytes on office broadband is typically tens of minutes, and a very large mailbox can take hours. Outlook stays usable while it downloads, so the wait is rarely dead time.

Will I lose my rules and settings if I rebuild the OST?

Server-side rules on Microsoft 365 and Exchange accounts live on the server and survive. Client-only rules and account settings live in the Outlook profile, not the OST, so a pure OST rebuild leaves them alone - another reason the rebuild beats profile surgery.

ScanPST repaired my OST and it worked. Was that wrong?

Not wrong, just unnecessary risk and time. A repair rewrites a cache that the server would have replaced for free. For an OST, rebuilding is faster to trust: every byte comes fresh from the server copy.

This guide is based on the Technical Anshu video above - if a step gets stuck, ask in the video comments and you will get an answer there.