When classic Outlook says it cannot open your data file or “the set of folders,” practical guidance is to preserve the existing file before attempting repair so you do not leave yourself with only a changed copy. Microsoft recommends backing up the existing Outlook data file before running its Inbox Repair tool.[1]
Guidance checked: October 7, 2026 at 1:03 p.m. EDT (17:03 UTC). This guide covers local Outlook data files in Windows. It does not promise recovery, replace a Microsoft 365 retention or backup plan, or authorize deleting a file until you know whether it is a replaceable cache or the primary copy.
First question: is it an OST cache or a PST data store?
One current Microsoft page says POP accounts store account data in a .pst, while Microsoft 365, Exchange, IMAP, and Outlook.com normally keep a mailbox copy in an offline .ost file.[4] A separate Microsoft page says classic Outlook POP or IMAP account information is stored in a PST, so Microsoft’s own pages differ on IMAP context.[2] Practical guidance: do not infer the file role from protocol alone; identify the exact file and account in Outlook’s Data Files list before changing it.[1] Microsoft says an Exchange .ost can be deleted and recreated the next time Outlook opens, but the Inbox Repair tool does not inspect the Exchange mailbox itself.[1] Practical guidance: do not apply that OST instruction to a PST. A PST may contain the only local copy of POP mail, archives, contacts, appointments, or tasks.[2][4]
Affected—or not affected?
Use this repair ladder
- Classic Outlook reports that it cannot open the data file or set of folders.[1]
- A known local PST will not open, or you suspect file corruption.[1]
- Practical guidance: distinguish a primary PST from a replaceable OST before changing anything.[1][4]
- You repaired a PST and now see Recovered Personal Folders or Lost and Found.[1]
Use a different path
- Webmail is also missing messages: practical guidance: investigate mailbox, retention, deletion, compromise, or provider issues—not only the local file, because Microsoft says SCANPST does not connect to or analyze data stored in an Exchange mailbox.[1]
- The drive reports read errors, disappears, clicks, or is failing health checks: practical guidance: stop write-heavy repair and preserve the storage evidence first; Microsoft describes SCANPST as checking Outlook data-file corruption, not repairing physical storage.[1]
- The PST is on a NAS, server share, or unstable mapped drive: Microsoft says long-term PST use over LAN/WAN is not the intended enterprise design.[3]
- You use new Outlook: PST capabilities and requirements differ; Microsoft says opening PSTs currently requires matching-architecture classic Outlook, and ARM new Outlook does not support PST files.[2]
The safe step-by-step fix
Close Outlook and record what failed
Practical guidance: write down the exact error, account name, Outlook version, whether webmail still shows the messages, and what changed immediately before the failure. Microsoft’s repair sequence begins by exiting Outlook; if Outlook remains in Task Manager after its window closes, do not copy or repair an actively changing file—allow it to exit cleanly or restart Windows before touching the data file.[1]
Identify the exact file and account
In classic Outlook, Microsoft directs users to File → Account Settings → Account Settings → Data Files to see the listed PST and OST files and the accounts associated with them.[1] Practical guidance: if Outlook will not open, preserve the error and ask a technician to map the Windows profile and Outlook profile before choosing a similarly named file; do not repair every PST or delete every OST found by a broad disk search.[1]
Make a separate copy before repair
Microsoft recommends backing up the existing Outlook data file before using SCANPST.EXE.[1] Practical guidance: with Outlook fully closed, copy the identified PST to a different healthy local or external drive, preserve its original filename and date, and verify that the copied file has the expected nonzero size. Keep the untouched original until repaired mail, contacts, calendar, and folders have been checked.
Stop condition and practical guidance: if the only PST is on a disk that produces read errors, disappears, or reports physical failure, do not keep rescanning or compacting it; preserve the storage through a qualified recovery path before logical Outlook repair. Microsoft describes SCANPST as checking Outlook data-file corruption, not repairing physical storage.[1]
Move an actively used network PST to a supported design
Microsoft says PST files are not intended as a long-term, continuous-use enterprise network store, and that remote file access adds overhead; Microsoft lists remotely accessed offline folders and address-book files as unsupported configurations too.[3] Practical guidance: do not run repair against a PST across Wi-Fi, VPN, NAS, or a mapped drive. Close Outlook, preserve a backup, copy the candidate to a healthy local fixed drive, and plan a mailbox or archive design instead of putting the live PST back on the share.[1][3]
Run Microsoft’s Inbox Repair tool on the copy you intended
Microsoft says SCANPST.EXE diagnoses and repairs errors in Outlook data files; its documented flow is to exit Outlook, open SCANPST.EXE, browse to the PST, start the scan, and choose Repair if errors are found.[1] Keep the repair option that creates a backup file enabled. Microsoft notes that multiple passes may be needed and that some permanently deleted or badly corrupted items may not be recovered.[1]
Inspect the recovered structure—not just whether Outlook opens
After repair, Microsoft says Outlook may show Recovered Personal Folders or a Lost and Found folder containing items the tool could not return to their original structure.[1] Practical guidance: compare folder counts and business-critical mail, contacts, calendars, and attachments against webmail, another device, or the preserved pre-repair copy. “Outlook opened” is not proof that every item or folder survived.
Build a clean successor when the repaired PST is questionable
Microsoft says you can create a new Outlook data file and move recovered items from Lost and Found into it.[1] Practical guidance: keep the original, SCANPST backup, repaired file, and clean successor as separate files until the new file passes an item-and-folder review. Do not overwrite the only pre-repair copy with the repaired result.[1]
Good and bad repair decisions
Good
- Confirm PST versus OST before deleting or rebuilding anything.[1][4]
- Close Outlook and copy the identified file before repair.[1]
- Practical guidance: repair a healthy local copy, not the only file on a network share.[1][3]
- Review Lost and Found and compare critical folders after repair.[1]
- Replace a fragile live-PST-on-NAS design with a supported mailbox or archive design.[3]
Bad
- Bad response: delete every file ending in OST or PST because one Outlook profile failed.[1][4]
- Bad response: run SCANPST repeatedly against the only copy while the drive is failing; Microsoft recommends backing up before repair and does not describe SCANPST as a hardware-recovery tool.[1]
- Bad response: rename a file extension and assume that changes its role or repairs it; identify the account and file through Outlook’s Data Files list instead.[1]
- Bad response: put the repaired live PST back on a NAS and share it between computers.[3]
- Bad response: assume a successful launch means all mail, contacts, and calendar items returned; Microsoft warns that some items may not be recoverable and explains Lost and Found.[1]
Two realistic examples
Recoverable cache problem
A Microsoft 365 user can read current mail in webmail, but classic Outlook fails after an unclean shutdown. The identified file is an OST. Practical guidance: after preserving account and profile details, a technician can rebuild the local cache because the mailbox remains on the server; this is different from deleting a PST that holds local-only data.[1][4]
Costly wrong turn
A long-used POP account has its mail, contacts, and calendar in one PST. Outlook will not open it, so the owner deletes the file after hearing that Outlook “rebuilds data files.” Microsoft’s account guidance distinguishes POP PST storage from server-backed OST cache storage.[4] Practical guidance: identify, copy, and repair that PST; do not treat it as a disposable cache.
After recovery: reduce the chance of a repeat
- Keep active PST files local. Microsoft says long-term network PST use is not the intended enterprise storage design.[3]
- Practical guidance: treat a PST export as a point-in-time file rather than automatic continuous protection, and test how you will restore the data you actually need.[2]
- Control size with supported tools. Microsoft says deleting items may not immediately shrink PST or OST files; Outlook can compact them in the background when unused space exceeds 20%, and users can start Compact Now manually.[4]
- Practical guidance: do not compact a questionable file on failing storage or use compaction as a substitute for the backup Microsoft recommends before repair.[1][4]
- Practical guidance: if a business still relies on POP or standalone PST archives, record the file location, owner, backup method, last restore test, and replacement plan.[2][4]
Frequently asked questions
Can I delete an OST file?
Microsoft says an Exchange OST can be deleted and Outlook will recreate it when Outlook opens again.[1] Practical guidance: first verify the account type, confirm needed data exists in the server mailbox, record the profile, close Outlook, and avoid applying the same step to a PST.
Can I delete a PST file?
Not as a generic repair step. Microsoft says PST files can contain messages, contacts, appointments, tasks, notes, and other Outlook items, and POP accounts may store all account information there.[2][4] Preserve and identify it first.
Does SCANPST guarantee recovery?
No. Microsoft says multiple passes may be needed and some permanently deleted or severely corrupted items may not be recoverable.[1]
Why did Lost and Found appear?
Microsoft says Lost and Found contains folders and items the repair tool recovered but could not place back into their original structure.[1] Review it before discarding any repaired file.
Can two computers use the same PST on a NAS?
That is not the normal supported design. Microsoft says PST files are not intended as a long-term enterprise network store and describes only a narrow hosted RDSH/VDI scenario with single-client access per file.[3] Use a mailbox or supported archive when multiple devices need the same data.
Related IT Guys guides
- Microsoft 365 deleted data has a clock: build a real backup and recovery plan beyond a local Outlook cache.
- Preserve a former employee’s Microsoft 365 or Google Workspace data before deleting the account.
- Test one file restore before trusting a backup—the same proof mindset applies to PST exports and archives.
Your next action
Practical guidance: before opening SCANPST or deleting an OST, write down four things: account type, file extension, exact file path, and where the authoritative mailbox data lives. Then make the backup that Microsoft recommends.[1][4]
Outlook still will not open, or are you unsure which file is safe to rebuild? Book an appointment with The IT Guys before changing the original file Microsoft recommends backing up.[1]