Outlook OST file corrupt? Fix "errors detected in the file" safely

What's actually wrong

Why Outlook says "errors detected" in your OST

Your .ost file (Offline Outlook Data File) is the local cache classic Outlook keeps on your PC so you can read mail when you're offline and so everything loads quickly. It is not your only copy. An .ost is only used by cached accounts — Microsoft 365, Exchange, Outlook.com and IMAP — and for those, your email also lives on the mail server. (One catch: on an IMAP account, classic Outlook keeps the calendar, contacts and tasks only inside the .ost, in folders marked (This computer only) — copy or export those before rebuilding.) The .ost simply mirrors it. (Older POP accounts don't use an .ost at all — they store mail in a .pst — so if Outlook is naming an .ost in the error, you're on a cached account and the server has the master copy.)

When Outlook reports "errors have been detected in the file .ost", that cache has become damaged — usually after Outlook or the PC was shut down mid-sync, an antivirus or backup program locked the file, the disk filled up, or the .ost simply grew huge over years of use. Because the master copy is safe on the server, the cleanest fix is to throw the damaged cache away and let Outlook build a fresh one. Nothing on the server is touched, so no email, contacts or calendar entries are lost.

Which repair route to use: Microsoft's Inbox Repair Tool (scanpst.exe) can technically scan an .ost, but for a corrupt .ost Microsoft's own advice is simply to delete it and let Outlook rebuild the cache from the server — that's the reliable, no-data-loss route. Save scanpst.exe for repairing a .pst (say, an old local archive), which is what it's really built for and where it earns its keep.

Stop it coming back

Exclude the OST from antivirus scanning

If the same error keeps returning, a common culprit is antivirus or backup software scanning the .ost while Outlook is writing to it. Real-time scanners can lock or half-read the file mid-sync and re-corrupt it. In your antivirus settings, add an exclusion for the Outlook data folder (%localappdata%\Microsoft\Outlook) or for the .ost file type. Windows Security, Bitdefender, Norton, McAfee and the rest all have an "exclusions" or "exceptions" area for this.

Also worth checking: make sure your disk isn't nearly full, and always close Outlook properly (don't just power the PC off) so the cache finishes writing. If you'd like, we'll set the exclusion correctly and confirm the mailbox is fully healthy while we're in — it's the kind of thing that takes us a couple of minutes and stops the nagging error for good.