Classic Outlook keeps freezing, the sync progress bar never finishes, or your folders show different item counts than Outlook on the web. These are common signs of a corrupted .OST file, and you can rebuild it by renaming the file so Outlook downloads a fresh copy, as long as you first back up any drafts that never reached the server.
How to Rebuild a Corrupted Outlook OST File: Quick Reference
Here’s the short version. Back up anything that exists only on your PC, find the .ost file in Account Settings > Data Files, close Outlook, rename the file to .ost.bak, and reopen Outlook so it rebuilds the cache from your Exchange or Microsoft 365 mailbox. IMAP accounts generally resync the same way (see Step 6).
| Situation | What to do |
|---|---|
| You have unsent drafts, Outbox items, or offline calendar edits | Step 1: Export them to a .PST backup first |
| You don’t know which .ost file belongs to your account | Step 2: Check File > Account Settings > Data Files |
| Windows won’t let you rename the .ost | Step 4: Confirm OUTLOOK.EXE isn’t running in Task Manager |
| Outlook freezes or sync is stuck | Steps 5–6: Rename to .ost.bak and relaunch Outlook |
| The rebuild didn’t fix crashing | Fix #1: Run Office Quick Repair or Online Repair |
| Problems survive both the rebuild and the repair | Fix #2: Create a new Outlook profile |
| You use the new Outlook for Windows app | This procedure doesn’t apply (see below) |
What Is an .OST File and Why Is It Safe to Rebuild?
An .OST (Offline Storage Table) file is Outlook’s local copy of your Exchange, Microsoft 365, or IMAP mailbox. It holds mail, calendar, contacts, and tasks so Outlook stays fast and keeps working when you’re offline. It is a cache, not the master copy. The server holds the real mailbox.
That’s why rebuilding works. Microsoft’s Repair Outlook data files (.pst and .ost) support page describes removing the damaged .ost so Outlook recreates it from the server. Microsoft’s documented process is deletion, not renaming.
This guide renames the file to .ost.bak instead, as an extra safety step this guide recommends rather than an official Microsoft-endorsed method. It isn’t required, but it gives you an undo option if something goes wrong before you’ve confirmed the rebuild worked.
Will you lose anything? Anything that already synced to the server comes back. Anything that lives only in the local cache won’t come back on its own. That includes drafts that never synced, messages stuck in the Outbox, and calendar changes you made while offline. Step 1 exists to protect exactly those items.
There’s a second, less obvious risk: when Cached Exchange Mode is on, classic Outlook may only have downloaded a limited window of older mail to begin with, the past 12 months by default according to Microsoft, though administrators can configure a different window. If older items were never cached, exporting “everything” in Step 1 won’t actually capture them, since they aren’t in Outlook’s local view to export in the first place.
Does this work in the new Outlook app? No. This guide covers Classic Outlook, the version with the File menu. Microsoft’s repair guidance applies to classic Outlook for Microsoft 365 and Outlook 2024, 2021, 2019, and 2016.
The new Outlook for Windows app uses a different, simplified interface, and it does not appear to offer the same user-managed Account Settings > Data Files path referenced in these steps. If your Outlook doesn’t show a classic File menu, check Microsoft’s version-identification guide below before following this procedure.
If you’re tidying up other parts of your Outlook setup while you’re in here, our guide on How to Change Your Profile Picture in Microsoft Outlook covers the account photo side of things.
Before You Begin
Make sure you have:
- A Windows 11 or Windows 10 PC
- Classic Outlook (such as Microsoft 365, Outlook 2021, or Outlook 2019) with an Exchange or Microsoft 365 account (IMAP accounts that use an .ost generally work too, with the caveats in Step 6; POP accounts normally keep mail in a .pst file instead, so this guide doesn’t apply to them)
- Outlook still able to open, even if it’s slow, so you can back up unsynced items
- Enough free disk space for Outlook to download your mailbox again
- A stable internet connection for the resync
- A place to save the backup outside the Outlook folder, such as your Documents folder or a USB flash drive
These steps are written for Windows 10 and Windows 11. Menu labels may differ slightly between Windows versions or Outlook update channels, so use the on-screen wording as your guide if it doesn’t match exactly.
| Requirement | Details |
|---|---|
| Device | Windows PC |
| OS version | Windows 10 or Windows 11 |
| Outlook version | Classic Outlook (Microsoft 365 / Outlook 2021 / Outlook 2019, or similar) |
| Account type | Exchange or Microsoft 365 (IMAP with an .ost generally similar, see Step 6) |
| Admin rights | Not required |
Not sure which Outlook you have? In Classic Outlook, go to File > Office Account to see your version details. If you don’t see a File menu, look for Settings > General > About Outlook. If that’s where you find it, you’re in the new Outlook. Microsoft’s What version of Outlook do I have? page shows both layouts.
Step 1: Back Up Unsynced Drafts and Calendar Items
Warning: Exporting in this step only backs up items Outlook can currently see. If Cached Exchange Mode is on, Outlook may have downloaded only a limited window of older mail by default, often the past 12 months, so items outside that window won’t be included even though they still exist on the server.
For anything you can’t afford to lose, confirm it also appears in Outlook on the web before continuing. If your organization’s policy allows it, you can also temporarily widen the cached window under File > Account Settings > Account Settings, then the account’s More Settings > Advanced tab.
Do this before you touch the .ost file. If Outlook can still open, you can save local-only items to a separate .PST file. Menu wording can vary slightly by Outlook build, but the general path below should match closely.
- Open Classic Outlook.
- Check your Outbox folder. If messages are sitting there, try Send/Receive > Send/Receive All Folders once. Anything that still won’t send needs to be backed up: drag each stuck message from Outbox into Drafts so it’s included in the Drafts export below. You can send it again once the rebuild is finished.
- Open your Drafts folder and note any drafts you can’t afford to lose.
- Go to File > Open & Export > Import/Export.
- Select Export to a file and click Next.
- Select Outlook Data File (.pst) and click Next.
- Select the Drafts folder. Leave Include subfolders checked, then click Next.
- Click Browse and save the file outside the Outlook data folder. For example, save
Drafts-backup.pstin your Documents folder. - Click Finish. If Outlook offers an optional password, you can skip it or set one you’ll remember.
- Repeat steps 4–9 for Calendar, and for Contacts if you’ve added people while offline. Give each export its own file name.
Expected result: Each export creates a .pst file in the location you chose. Open that folder in File Explorer to confirm the files exist.
Then open one of the backups with File > Open & Export > Open Outlook Data File and spot-check that a few of your important drafts and calendar items are actually inside. A .pst that exists isn’t proof that everything was captured. Close the backup data file before moving on.
Tip: To grab everything Outlook currently has cached, pick the top level of your mailbox in step 7 with Include subfolders checked. The cached-window limit from the warning above still applies. The file will be much larger and the export will take longer, so drafts plus calendar is usually enough.
Step 2: Find the Active .OST File in Account Settings
Don’t guess which file to rename. Outlook can have several .ost files, especially if you’ve set up the same account more than once.
Menu wording for Account Settings can vary slightly across Outlook versions, but the Data Files tab should be in roughly the same place. The path shown there is the most reliable way to identify the file for the account you’re fixing.
- In Outlook, go to File > Account Settings > Account Settings.
- Click the Data Files tab.
- Select the account that’s having problems. The Location column shows the full path to its .ost file.
- Write down the file name. It’s usually your email address followed by
.ost. - Click Open File Location. File Explorer opens to the folder containing the file, with the file selected.
- Click Close in the Account Settings dialog. Leave the File Explorer window open.
Expected result: You know the exact file name and have File Explorer open to its folder.
The folder is often %LOCALAPPDATA%\Microsoft\Outlook. You can paste that into the File Explorer address bar or the Run box (Win + R). Your setup may store the file elsewhere, though. Always trust the path shown on the Data Files tab.
Step 3: Turn On File Name Extensions
Windows often hides file extensions. If you can’t see .ost at the end of the file name, you can’t rename it correctly.
- In the File Explorer window from Step 2:
- On Windows 11: Click View > Show > File name extensions.
- On Windows 10: Click the View tab on the ribbon and check File name extensions.
- Confirm the file now ends in
.ost.
Expected result: The full file name, including .ost, is visible.
Step 4: Close Outlook Completely
Outlook keeps the .ost locked while it’s running. Closing the window isn’t always enough, because Outlook can keep running in the background. In most installations, Task Manager lists this process as OUTLOOK.EXE, though exact labeling can vary slightly by version.
- In Outlook, click File > Exit. You can also close the window normally.
- Wait about 30 seconds for Outlook to finish writing to its files.
- Press
Ctrl + Shift + Escto open Task Manager. - On the Processes tab, look for Microsoft Outlook. For a precise check, open the Details tab and look for
OUTLOOK.EXE. - If it’s still listed after you closed Outlook normally, select it and click End task.
You can also check from Command Prompt:
tasklist /fi "imagename eq outlook.exe"Expected result: Task Manager shows no Outlook process, or the command returns INFO: No tasks are running which match the specified criteria.
Warning: Only end the task after you’ve tried to exit normally. Killing Outlook mid-write is one way .ost files get corrupted in the first place.
Step 5: Rename the .OST File to .ost.bak
Rename the file. Don’t delete it. The renamed copy is your safety net.
- In File Explorer, click the .ost file you identified in Step 2.
- Press
F2. - Add
.bakto the end of the name. For example,[email protected]becomes[email protected]. - Press
Enter. - If Windows warns “If you change a file name extension, the file might become unusable,” click Yes.
If you prefer Command Prompt, use ren. Replace the file name with your own:
ren "%LOCALAPPDATA%\Microsoft\Outlook\[email protected]" "[email protected]"Expected result: The file now ends in .ost.bak, and no file with the original .ost name remains in the folder.
To undo, close Outlook, delete any new .ost that Outlook created, and rename the .bak file back to .ost.
Step 6: Relaunch Outlook to Trigger a Fresh Resync
- Open Outlook as you normally would.
- Sign in if Outlook prompts you.
- Outlook can’t find the .ost it expects, so it creates a new one and starts downloading your mailbox from the server.
- Watch the status bar at the bottom of the Outlook window for sync progress. Folders will fill in gradually.
- Leave Outlook open and connected until the status bar shows syncing is finished.
How long does it take? It depends on mailbox size, connection speed, and how much mail Outlook is set to keep offline. A small mailbox may finish in minutes. A large one can take hours. Plugging a laptop into your router with a Cat6 Ethernet cable is usually faster and more reliable than Wi-Fi for a big download.
IMAP accounts: This guide’s steps are based on Microsoft’s documented behavior for Exchange accounts; IMAP behavior isn’t addressed in the same official guidance. The general resync process should still apply, since Outlook rebuilds its cache from whichever server it’s connected to either way. Compare folder counts carefully in Step 7 to confirm nothing is missing.
Step 7: Verify the Rebuild and Restore Your Drafts
Confirm the rebuild worked, then bring back anything you backed up. Menu names below may vary slightly depending on your Outlook version.
- Go back to the Outlook data folder in File Explorer. You should see a new
.ostfile alongside your.ost.bak. - Open Outlook on the web or your mail provider’s webmail. Compare the item counts of a few key folders, such as Inbox and Sent Items, against Classic Outlook.
- Confirm the freezing or stuck sync is gone.
- To restore backed-up drafts, go to File > Open & Export > Open Outlook Data File.
- Browse to your backup .pst (for example,
Drafts-backup.pst) and click OK. It appears as a separate folder group in the navigation pane. - Expand it, open its Drafts folder, and drag only the drafts that are missing into your mailbox’s Drafts folder. For the calendar, restore only events you created or changed while offline that didn’t come back after the resync. Copying the whole backed-up calendar back would duplicate events that already synced from the server.
- When you’re done, right-click the backup data file in the navigation pane and choose Close “[file name]”.
Expected result: Your folders match the server for everything inside your cached window (older mail outside a shortened window stays server-only), and local-only drafts are back where you left them.
Keep the .ost.bak file for a few days. Once you’re sure nothing is missing, you can delete it to free up disk space.
Configuration: Settings That Affect the Rebuild
| Setting | Where to find it | Why it matters |
|---|---|---|
| Data Files tab | File > Account Settings > Account Settings > Data Files | Shows every .ost and .pst tied to each account. This is the most reliable way to pick the right file. |
| Cached Exchange Mode slider | File > Account Settings > Account Settings, then double-click the account (on some versions, also More Settings > Advanced) | Controls roughly how many months of mail stay available offline; a smaller window may speed up the rebuild. |
| File name extensions | File Explorer View menu or tab | Must be on so you can see and edit the .ost extension. |
Microsoft documents a 12-month default download window for Cached Exchange Mode, but administrators can configure a different one, so yours may differ from a coworker’s. If a rebuild is crawling, temporarily dragging the slider to a shorter window gets your recent mail working first. Older mail outside that window stays on the server only, so expect lower folder counts than Outlook on the web in Step 7. You can widen it again later. At work, your IT admin may have locked this setting.
Fix #1: Run an Office Quick Repair or Online Repair
If Outlook still crashes or corrupts its new .ost, the problem may be a damaged Office installation rather than the cache file.
- Close Outlook and all other Office apps.
- Open the Office entry in Settings:
- On Windows 11: Go to Settings > Apps > Installed apps, find your Microsoft 365 or Office entry, click the three-dot menu, and choose Modify.
- On Windows 10: Go to Settings > Apps > Apps & features, select your Microsoft 365 or Office entry, and click Modify.
- Click Yes if User Account Control asks for permission.
- Choose Quick Repair and click Repair.
- When the repair finishes, open Outlook and test it.
- If problems continue, repeat the process and choose Online Repair. It needs an internet connection and takes longer, but it’s more thorough.
Fix #2: Create a New Outlook Profile
A new profile rebuilds Outlook’s entire local configuration, including the .ost. Use it when the rename and the repair both fail.
- Close Outlook.
- Open Control Panel. Set View by to Large icons or Small icons, then click Mail (Microsoft Outlook). The label may vary slightly by version.
- Click Show Profiles.
- Click Add, type a new profile name, and click OK.
- Follow the prompts to sign in to your email account.
- Back in the profiles dialog, select Prompt for a profile to be used. This lets you switch between the old and new profiles while you test.
- Click OK, open Outlook, and choose the new profile.
Expected result: Outlook builds a brand-new profile and .ost from the server. Once the new profile works, set it as the default with Always use this profile. Remove the old profile only after you’re confident nothing is missing.
Tips and Troubleshooting
Windows won’t let me rename the .ost file
Why it happens: OUTLOOK.EXE, or a related background process, still has the file open.
Fix: Repeat Step 4 and confirm Outlook isn’t running. If something else keeps the file locked, restart Windows. Don’t open Outlook afterward, and rename the file right after you sign in.
I can’t see the .ost extension
Why it happens: Windows hides known file extensions by default.
Fix: Turn on File name extensions (Step 3). On Windows 10 you can also go to View > Options > View tab and uncheck Hide extensions for known file types.
Outlook didn’t create a new .ost
Why it happens: The file was copied instead of renamed, you renamed a different account’s file, or Outlook opened a different profile.
Fix: Recheck the exact path on the Data Files tab. Make sure the original .ost name no longer exists in that folder, and confirm you’re launching the same profile.
My drafts or recent calendar edits are gone
Why it happens: They existed only in the local cache and never synced before the rename.
Fix: Restore them from the .pst you exported in Step 1. Microsoft’s guidance doesn’t promise recovery of unsynced content from a rebuilt cache. If you skipped the backup, close Outlook and restore the .ost.bak (see the undo note in Step 5), then export the items before trying again.
Someone told me to run scanpst.exe on the .ost
Why it happens: It’s easy to confuse the two repair approaches.
Fix: Microsoft’s documented repair steps using scanpst.exe (the Inbox Repair Tool) apply to .PST files. For a damaged Exchange .OST, Microsoft’s official guidance is to delete or rename the file so Outlook rebuilds it, not to run scanpst.exe against it. Use the rename-and-rebuild method above instead.
My disk filled up during the resync
Why it happens: Large mailboxes need a lot of space, and the .ost.bak file is taking up space too.
Fix: Shorten the Cached Exchange Mode window, or move the .ost.bak to a USB flash drive until you’ve verified the rebuild. If your drive is constantly full, an SSD upgrade is worth considering. Outlook caches tend to grow over time.
Wrapping Up
When a damaged cache is the cause, renaming the .ost and letting Outlook rebuild it usually clears freezing and stuck-sync problems in Classic Outlook. Add-ins, profile problems, connectivity, or server issues can cause similar symptoms, which is what the repair and new-profile fixes are for.
Skipping the backup is the biggest avoidable risk, so treat Step 1 as non-negotiable and keep the .ost.bak until you’ve confirmed nothing is missing. If a fresh .ost corrupts again within days, or Outlook crashes even with a new profile, look beyond Outlook. Check your drive’s health, especially if other apps hang too, and run a malware scan before blaming Office.
| Step | Action | Applies To |
|---|---|---|
| 1 | Move stuck Outbox items to Drafts, then export drafts and calendar items to .pst | Anyone with unsynced items |
| 2 | Find the .ost via Data Files > Open File Location | Classic Outlook accounts that use an .ost |
| 3 | Turn on file name extensions | Windows 10 and 11 |
| 4 | Exit Outlook and confirm no OUTLOOK.EXE | All |
| 5 | Rename .ost to .ost.bak | All |
| 6 | Relaunch Outlook and let it resync | Exchange and Microsoft 365 (IMAP generally similar) |
| 7 | Verify counts and restore drafts | All |
| Fix #1 | Quick Repair, then Online Repair | Ongoing crashes |
| Fix #2 | New Outlook profile | Problems that survive everything else |