What is an OST File And Why It Cannot Simply Be Copied

An OST is a cache rather than a copy. It exists so Outlook stays usable when the network is not, it holds whatever the sync settings decided was worth keeping locally, then carries a record of which mailbox it belongs to. That last detail is the whole reason moving one to another machine achieves nothing.

Offline Storage Table is the full name. Microsoft has also called the same thing an Offline Folders file. Almost nobody uses either. What people do instead is find a large file with an unfamiliar extension, copy it somewhere safe, then discover that nothing will open it. The file is fine. The refusal is deliberate. Understanding why makes the rest of the format straightforward. Unsure whether an OST is even what you have? The format identification table settles it from the first line of the file.

What an OST Is Actually For

Outlook can talk to a server mailbox two ways. Online mode fetches each message as you click it, which needs a connection for everything. Cached Exchange Mode keeps a local copy and works from that, syncing changes back when it can. The OST is that local copy. What it copies from is a mailbox inside an Exchange database on the server, so an OST is a cache of one slice of an EDB.

The point is responsiveness rather than backup. Reading from a file on your own disk is faster than a round trip to a mail server, so Outlook feels quick even on a slow link and keeps working on a train. Everything else about the format follows from being built for that job rather than for storage.

Why the File Refuses to Move

Copy an OST to a memory stick, carry it to another machine, point Outlook at it. Outlook answers with a message about the file being configured for use with a different mailbox. People read that as corruption. It is not.

An OST records which mailbox it caches. Outlook profiles are MAPI profiles, so each account in one carries an identity for the mailbox behind it. Opening the cache means comparing the two. A cache is only valid for the thing it caches, so a mismatch means the file is meaningless to this profile and Outlook says so rather than attaching a store full of somebody else's mail.

open the OST
read the mailbox it caches
compare to the profile account
same mailbox
attach the store

open the OST
read the mailbox it caches
compare to the profile account
different mailbox
refuse

Outlook has no import for OST. The File menu will import a PST all day. It will not import an OST in any version, because an OST is not something the format was ever meant to accept from outside. The route out of one is to read it with software that parses the file directly.

What an OST Does Not Contain

Here is where the cache word earns its keep. An OST holds what Outlook decided to keep locally, which is not the same as what the mailbox holds.

Cached Exchange Mode has a setting called Mail to keep offline. Microsoft documents the desktop default as twelve months, with the slider running from three days at one end to All at the other. Anything outside that window stays on the server and never reaches the file.

So a fifteen gigabyte OST taken from a sixty gigabyte mailbox is not a broken copy. It is a correct cache of the last twelve months, behaving exactly as configured.

Check the window before assuming the file is complete. Account settings, double click the account, read the Mail to keep offline slider. Whatever it says is what the OST contains, however large the file looks or however old the account is. On Outlook 2016 and later the same slider governs cached shared folders too. In managed estates the value often comes from Group Policy rather than from the user, so the slider may be set and locked.

The Encryption Myth

A number of pages selling recovery software describe an OST as server side encrypted, giving that as the reason you cannot open one elsewhere. It is wrong on both counts.

An OST is built to the same specification as a PST, down to the same B-tree indexes and the same block layout. It uses the same keyless encoding too, which Microsoft describes in that specification as obfuscation providing no additional security. No key exists. Nothing is decrypted. Software that reads the format reads an OST exactly as it reads a PST.

The reason Outlook refuses a copied OST is the mailbox check described above, which is a policy decision rather than a cryptographic one. One of those can be worked around by reading the file directly. The other could not be.

What Happens When the Mailbox Goes Away

While the mailbox exists, the OST is disposable. Delete it and Outlook builds another. That reverses completely the moment the account stops existing.

An employee leaves and the mailbox is deprovisioned. A company closes an account. A subscription lapses. From that point the OST on the laptop is not a cache of anything, because the thing it cached is gone. It becomes the only remaining copy of whatever it happened to be holding.

The desktop looks identical either way. A working cache and a cache of a mailbox that no longer exists present the same file, the same size and the same folder tree. Nothing warns you which one you have, which is exactly why OST files get deleted during a clear out at the worst possible moment.

too large   shorten the window
damaged     delete it, resync
wrong size  ignore it
moving PC   leave it behind
deleted     rebuilds itself

too large   leave it alone
damaged     recover, never repair
wrong size  that is all there is
moving PC   take it with you
deleted     gone permanently

The Size Limit And Why Raising It Is the Wrong Move

An OST carries the same ceiling a PST does, set through the registry and defaulting to fifty gigabytes on any recent Outlook. Hit it and Outlook stops writing to the file.

Virtual desktop estates hit this first, since a profile container has its own ceiling and the OST sits inside it. The usual response is to look up the registry key and raise the number. For a PST that is at least arguable, since a PST is the only copy of what it holds. For an OST it is close to the worst available answer, because the sync window is sitting right there.

All of that assumes the left column above. Shortening the window trims the cache safely because the server still keeps the original. Do the same thing to an OST whose mailbox has already gone and the trimmed mail leaves with it, since there is nothing left to sync it back from.

Whether an OST Can Be Repaired

Search for a damaged OST and the results disagree with each other. Some say the Inbox Repair Tool, scanpst.exe, fixes one. Others say it was never built for the format and will not touch it. Both positions have supporters writing confidently.

The disagreement matters less than it looks, because for a working account the question rarely needs answering. A cache with a problem does not need repairing. It needs deleting, at which point Outlook builds a clean one from the server and the argument becomes academic.

While that rebuild runs, the mail has not gone anywhere. Outlook on the Web shows the same mailbox from the server side, which is worth remembering during the hour a large mailbox takes to download again.

Repair only becomes a real question in the right hand column. With nothing to resync from, the damaged file is the only copy, so a tool that reads the format directly is doing recovery rather than repair. That is a different job from the one the Inbox Repair Tool was written for.

Why New Outlook Has No OST at All

Microsoft has confirmed in its own answers forum that the new Outlook for Windows does not import OST files, because it was built on a different architecture. It talks to the mailbox directly instead of maintaining a local store.

Two things follow from that. Anybody holding an OST today got it from classic Outlook, so the file already belongs to a previous generation of the product. The population of these files is also fixed rather than growing, since the client that produced them is being replaced.

If an old one is sitting on a disk somewhere, that is a reason to open it now rather than later. The tooling that reads these files is not going to improve. The client that created them is on its way out.

What This Means If You Are Holding One

1. Find out whether the mailbox still exists. Everything depends on this. If it does, the OST is disposable and the mail is safe on the server. If it does not, you are holding the only copy and it needs treating that way.
2. Check the sync window before trusting the contents. A large file is not a complete file. The account settings tell you how far back Outlook was keeping mail.
3. Do not expect Outlook to help. No version imports an OST. Time spent on new profiles and repair tools is time spent on the wrong problem.
4. Read it directly instead. The format is documented, so software outside Outlook can open the file without any profile at all.

An OST that has outlived its mailbox is worth opening sooner rather than later, since nothing about the situation improves with time. The viewers take the file as it is, with no profile and no Outlook involved, which answers the question of what is actually in there. If any of it needs to live somewhere usable afterwards, that is what the converters are for.