Repair Corrupted Outlook .pst/.ost Data File

Outlook stores mailbox data locally in two file types: .pst (Personal Storage Table — used for POP accounts, archives, and imported data) and .ost (Offline Storage Table — a cached copy of an Exchange/Microsoft 365 mailbox). When these files become corrupted due to abrupt shutdowns, disk errors, oversized files, or synchronization conflicts, Outlook typically fails to open with errors such as:

• "The file <path>.ost is not an Outlook data file (.ost)."
• "Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened."
• "Errors have been detected in the file. Quit Outlook and all mail-enabled applications, and then use the Inbox Repair Tool."

This recipe walks through repairing or rebuilding the affected data file. Because .ost files are merely a local cache of a server-side mailbox, the safest remedy is usually to delete and let Outlook rebuild them. .pst files, however, contain authoritative data and must be repaired with scanpst.exe (the Inbox Repair Tool).

• Outlook launches successfully without data-file errors.
• Corrupted .ost files are rebuilt from the server mailbox (no data loss for Exchange/M365 accounts).
• Corrupted .pst files are repaired with scanpst.exe and a backup preserved.
• You have a documented path for recurring corruption (profile recreation, OST size monitoring).

Important warnings

• Always back up the .pst file before running scanpst.exe. The tool modifies the file in place and can occasionally make corruption worse.
• Do NOT delete a .pst file without confirming its contents are archived elsewhere — .pst data is not stored on the server.
• Deleting an .ost is safe only when the account is an Exchange, Microsoft 365, or IMAP account that syncs from the server. Verify account type first.
• Very large .ost files (>50 GB) are a common cause of recurring corruption. Consider reducing cached mail range (e.g., to 12 months) after recovery.
• Running scanpst.exe on a file currently in use by Outlook will fail — close Outlook completely (check Task Manager for OUTLOOK.EXE) before starting.
• If the user has Personal Archive or Online Archive mailboxes, those are separate and not affected by local file repair.