What Is an OST File? Outlook's Offline Copy of a Mailbox That Lives on a Server
An OST file is the offline Outlook Data File that caches your mailbox on your own computer. Jump to your question below, or read it through in order.
An OST file is the offline Outlook Data File that caches your mailbox on your own computer. Jump to your question below, or read it through in order.
An OST file is an offline Outlook Data File. Outlook builds one for an Exchange, Microsoft 365, Outlook.com or IMAP account. It keeps a synchronized copy of the mailbox inside that file, so you can read and write email with no connection.
Windows stores it in AppData\Local\Microsoft\Outlook. It caps at 50 GB on Outlook 2010 and later. It is a cache, not a backup, so Microsoft says you never need to back it up.
Think of an OST file as a shadow of your mailbox. The real mailbox sits on a mail server. Outlook keeps a second copy on your hard drive so the app stays usable on a plane, on a bad hotel connection or while the server is down.
Everything you see in Outlook appears in that copy: email and attachments, the folder tree, contacts, calendar events, tasks and notes. When the connection returns, Outlook sends up whatever you changed and pulls down whatever arrived.
The naming trips people up. Almost every guide expands OST as Offline Storage Table, and that is the phrase most people search for. Microsoft does not use it. In Outlook's own menus and in Microsoft support pages the file is an offline Outlook Data File. Both names mean a file ending in .ost.
New Outlook for Windows does not use an OST file. The rebuilt app keeps its offline data in a different store, so the paths and fixes on this page apply to classic Outlook. If your Outlook has no File tab, you are in new Outlook.
Outlook creates an OST file whenever an account keeps its mail on a server and Cached Exchange Mode is on. That covers Exchange, Microsoft 365, Outlook.com and IMAP.
Outlook could talk to the server for every click. It does not, because that feels slow and it breaks the moment the network does. Instead it turns on Cached Exchange Mode and works from the local copy, syncing in the background.
That local copy is the OST file. It is created the first time the account is set up, which is why a brand new profile spends its first few minutes downloading before the mailbox looks complete.
| Account type | Where your email lives | Which file Outlook makes |
|---|---|---|
| Microsoft 365 or Exchange | Server, cached locally | OST |
| Outlook.com | Server, cached locally | OST |
| IMAP, Outlook 2016 and later | Server, cached locally | OST |
| IMAP, Outlook 2013 and earlier | Server, with a local copy | PST |
| POP3 | Downloaded to your computer | PST, and it is the only copy |
The last row is the one worth remembering. A POP account has no server copy to fall back on, so it writes to a PST file instead. Everything else on that list keeps the original on the server and treats the local file as disposable.
An OST file is a cache that the server can refill. A PST file is often the only copy there is. Lose an OST and you lose nothing. Lose a PST and the email may be gone.
Both files end up in the same folder tree in Outlook and both hold the same kinds of item. That is where the similarity stops. The difference that matters is where the original lives.
A copy. The original sits on the mail server. Outlook can throw the file away and download a fresh one whenever it needs to.
An original. For a POP account or an archive, nothing else holds that email. Delete the file and it is gone for good.
| Question | OST file | PST file |
|---|---|---|
| Who creates it | Outlook, automatically | You, or an export or archive job |
| Default folder | AppData\Local\Microsoft\Outlook | Documents\Outlook Files |
| Needs a backup | No | Yes, it is often the only copy |
| Opens on another computer | No, it is tied to one mailbox | Yes, any classic Outlook opens it |
| Attach it by hand | Not possible | File, Open & Export, Open Outlook Data File |
| Safe to delete | Usually, Outlook rebuilds it | Only if the email exists elsewhere |
| Default size cap | 50 GB | 50 GB |
Every row after the first falls out of one fact. A server holds the original for an OST file, so the file is expendable. Nothing holds the original for most PST files, so the file is precious. Read the rest of this page with that in mind and the advice stops feeling arbitrary.
If you want the other half of the story, our guide to the PST Outlook Data File covers how to open, shrink and repair one.
On Windows 10 and Windows 11 the default folder is C:\Users\<username>\AppData\Local\Microsoft\Outlook. AppData is hidden, so paste the path into File Explorer.
The OST file lives somewhere different from the PST file, and mixing the two up is the single most common mistake in older guides. A PST file lands in your Documents folder. An OST file lands in a hidden system folder.
The file is normally named after the account, such as you@company.com.ost. A profile with two server accounts has two OST files sitting side by side, one for each.
Do not move an OST file by hand. Outlook records the path in the profile. Move the file and Outlook simply builds a new one in the old place. To relocate it properly, an administrator sets the ForceOSTPath registry value before the account is created.
Outlook 2016 changed how it resolves file paths, starting with build 16.0.7910.1000. From that build on it refuses to add a .pst or .ost file that is not on a local hard drive. A mapped network drive, a NAS share or a removable disk all fail.
The errors you get are misleading. Outlook reports an invalid file name, or access denied, neither of which hints that the location is the problem. Keep the file where Outlook puts it.
There are other files in that folder. A matching .nst file holds the search index and group data for the same account, and an .oab file caches the address book. Leave both alone. Outlook rebuilds them whenever it rebuilds the OST file.
An OST file defaults to 50 GB on Outlook 2010, 2013, 2016 and later. On Outlook 2003 and 2007 the default is 20 GB. Outlook starts warning at 47.5 GB and 19 GB.
An OST file grows to match the mailbox it mirrors. Outlook applies a ceiling anyway, and it warns you well before the file reaches it so its own clean-up jobs have room to work.
| Outlook version | Maximum size | Warns at |
|---|---|---|
| 2010, 2013, 2016 and later | 50 GB | 47.5 GB |
| 2003 and 2007 | 20 GB | 19 GB |
Hit the ceiling and Outlook stops writing. The message names the file rather than the limit, and the code behind it is 0x8004060C. On an OST the fix is almost never a bigger cap. It is less mail kept offline.
An administrator can change both numbers. The values are MaxLargeFileSize and WarnLargeFileSize, and they are written in megabytes, not bytes.
Microsoft asks for a gap of at least 5 percent between the maximum value and the warning value, so internal processes are not blocked. Setting the warning to the same number as the maximum is a common and avoidable mistake.
Most people assume their own mail filled it. Often it did not. Add somebody else's shared mailbox to your profile and Outlook downloads every folder you have access to into your OST file, not into a separate one.
Microsoft says so directly. If the shared folders hold many items or large attachments, your file can grow a lot. Caching shared folders can be what pushes it into the size limit.
From Outlook 2016 the Download email for the past slider applies to shared mailboxes as well, which keeps this in check. An administrator can switch that off with a registry value named DisableSyncSliderForSharedMailbox. If it is set to 1, the whole shared mailbox syncs to your disk.
So before you go hunting for large attachments in your own Inbox, count the shared mailboxes in your folder list. Removing one you no longer need can free up space faster than anything else on this page.
Raising the cap is rarely the right fix. A very large OST file makes Outlook slow to search and slow to start, long before it hits any ceiling.
Reduce how much mail Outlook keeps offline instead. Go to File, then Account Settings, then Account Settings, double-click the account and move the Mail to keep offline slider down. The server keeps everything either way.
Emptying Deleted Items and clearing large attachments helps too, but only after the next sync. The file shrinks when Outlook compacts it, not the moment you press delete.
No. Microsoft states that offline Outlook Data Files are copies of your Exchange information. It says backing up this file is not necessary and that you can create a new copy at any time.
This is the question behind most OST searches, even when people ask it another way. Can I recover my email from this file? Should I copy it before I reinstall? Why will it not open on the new laptop?
Microsoft answers it directly. Its support page describes them as copies of items already saved on a mail server. The same page says they "don't need to be backed up like Outlook Data Files (.pst)".
So the file on your disk is scaffolding. The building is the mailbox on the server. Back up the mailbox, or trust your provider's retention. Either way the OST file stops mattering.
There is one case where the file does hold something unique. If somebody dragged email into a local folder while offline, or the account was closed before the last sync finished, that content may exist only in the OST file. Deal with it before the account goes away, by exporting a PST file while Outlook still opens.
You do not open an OST file the way you open a PST file. It opens by itself when its own Outlook profile starts. There is no Open Outlook Data File route for it.
Double-clicking the file in File Explorer does nothing useful. Windows has no handler for the format, so you get an error or a dialog asking which program to use. Outlook's own Open Outlook Data File command refuses it as well, because that command is built for PST files.
The supported way is simply to start Outlook with the profile that owns the file. Everything inside it appears in the normal folder tree.
Copy the file to a second machine and Outlook turns it away. The wording is exact, and it is worth recognizing:
"The file <path>.ost cannot be accessed because it has been configured for use with a different mailbox."
Outlook stamps the file with the mailbox that created it. A different profile, even for the same person, counts as a different mailbox. Add the account on the new computer and let Outlook build a fresh file there.
No other mail app reads the file either. Gmail, Thunderbird, Apple Mail and Outlook for Mac all expect a different format. None of them can mount an OST. If you need the mail somewhere else, connect that app to the same account. Or export the mailbox to a PST file first.
If Outlook refuses the file on the machine that made it, the error usually points somewhere else. The Outlook permission denied error means the file is read-only or locked by another program. Our guide to new Outlook errors covers the rebuilt app, which does not use an OST file at all.
Open the account in Outlook, then use File → Open & Export → Import/Export → Export to a file and choose Outlook Data File (.pst). No extra software needed.
Use this when you are leaving a job, closing an account or want a copy of the mailbox you can keep.
Almost every page about converting an OST file sells a converter. You rarely need one. While the account still opens in Outlook, the export built into Outlook produces a normal PST file from exactly the same data. That is true whether you want one folder or a full migration off the account.
AppData, such as your Documents folder, then click Finish.The result is an ordinary PST file. Any classic Outlook on any computer can open it, which is precisely what the OST file could not do.
Do the export before the account is switched off. A converter tool is only worth buying when the profile is already gone and the orphaned file is all that is left.
Even then, check what you actually need. If the mailbox still exists on the server, connecting to it from a fresh profile gets the email back with no tool and no risk.
SCANPST.EXE repairs an OST file as well as a PST file. On an OST it is usually the slower answer, because deleting the file and resyncing gives you a clean copy.
A damaged OST file shows up as folders that will not open, a sync that never finishes or Outlook freezing on start. The Inbox Repair Tool that ships with Office does accept .ost files, so it is a valid fix.
It is rarely the fastest one. Repair works through the file block by block and can take hours on a large mailbox, and it may still drop whatever it cannot read. The server already has a clean copy, so the rebuild in the next section usually wins.
That is the part worth holding on to. On a PST file, corruption is an emergency, because the damaged file may be the only copy. On an OST file it is an inconvenience. The email is safe on the server either way.
Use this when the account is gone and the orphaned file is the only copy you have.
OUTLOOK.EXE.SCANPST.EXE in the Office folder. Typical path: C:\Program Files\Microsoft Office\root\Office16..ost file.If Outlook reports a specific code rather than a vague failure, start there instead. Outlook error 0x80040600 points at damage inside the data file, and our Outlook PST import and export error guides cover the rest.
Close Outlook, rename the .ost file, then start Outlook again. It builds a fresh file and downloads the mailbox from the server.
Because the server holds the original, throwing the local file away is a normal repair rather than a risk. Rename instead of deleting, so you can put it back if something was not synced.
C:\Users\<username>\AppData\Local\Microsoft\Outlook in File Explorer.you@company.com.ost to you@company.com.ost.old..old file.On a metered or slow connection, remember that a rebuild pulls the whole mailbox down again. Doing it on a phone hotspot is an expensive way to fix a sync glitch.
Rename, do not delete, on the first attempt. Anything that was dragged into a local folder while offline lives only in that file. Keeping it as .old for a week costs nothing and has saved plenty of people.
An OST file uses the same on-disk container as a PST file, and Outlook reaches both through MAPI. The difference is the sync state stamped inside, which binds the file to one mailbox.
You do not need this section to use an OST file. It is here because two things make more sense once you know them.
The first is why repair tools work on the index rather than on the email. The second is why a file that looks like an ordinary document refuses to travel between computers.
.ost.nst, the search index for the same accountSCANPST.EXE, the Inbox Repair ToolThis is worth saying plainly, because most pages about OST internals imply more than anyone knows. Microsoft publishes a full specification for the PST file, called [MS-PST]. It has never published one for the OST file.
So the honest position is this. The PST format is documented. The OST format is not. What we can say about the OST comes from how Outlook's own tools behave. The same repair tool opens both. The same registry key caps both. Both are reached through the same interface.
The published PST spec describes three layers stacked on each other. It is a useful picture even though it describes the sibling format.
Outlook never reads the file end to end. It looks a message up in the index and jumps straight to the block that holds it. That is what makes a 40 GB file usable at all.
It also explains the repair behavior. Damage near the index and Outlook loses its map of the whole file, even though almost every message is still intact. Repair tools rebuild the map, which is why they report far more errors than the amount of email actually lost.
The binding to a mailbox is not part of the container. It is recorded state, written when the account is set up. That is why no amount of copying or renaming persuades another profile to open the file.
Outlook reaches the data through MAPI, the same interface it uses for a PST file. That shared plumbing is why SCANPST.EXE handles both, and why the two formats hit the same registry key for their size limits.
Is Outlook refusing to sync, or showing a code you cannot place? Work through the matching guide in our Outlook data file error guides. Nothing matching? Send us the exact wording on screen and we will write it up.