OST vs PST in Outlook: what’s the difference, and how to get your mail out

What is a PST file?

A PST (Personal Storage Table), which Outlook calls an Outlook Data File, is a self-contained database of Outlook items: email, contacts, calendar, tasks and notes, in a full folder tree. Nothing about it depends on a server. You can copy it to a USB drive, open it on another computer or attach it to a different Outlook profile.

That makes PST the file you use when mail needs to exist on its own:

• POP accounts. A POP account downloads mail from the server and keeps it in a PST, so the PST is the only copy.
• Archives and backups. Classic Outlook’s archive feature and the Import/Export wizard both write PST files.

For how PST compares with the formats other mail apps use, see MBOX vs PST.

What is an OST file?

An OST is an Offline Outlook Data File. Classic Outlook creates one for accounts where the real mailbox is on a server: Microsoft 365 and Exchange, Outlook.com, and IMAP accounts. The OST is a synchronized copy of that mailbox, so you can read and search mail quickly, and keep working when you are offline. Changes you make sync back to the server when you reconnect.

Cached Exchange Mode

For Microsoft 365 and Exchange accounts, the OST comes from Cached Exchange Mode, which is on by default. Cached mode doesn’t necessarily copy your whole mailbox. A setting called Download email for the past (called Mail to keep offline in some versions) decides how much mail is kept locally. Microsoft documents a default of 12 months on most computers, and as little as 1 or 3 months on devices with small disks. Older mail stays on the server, and Outlook shows a link to view it.

Change it in classic Outlook under File → Account Settings → Account Settings: on the Email tab, double-click the account and move the slider, all the way to All if you want everything cached. Outlook asks you to restart to apply the change.

Where Outlook stores OST and PST files on Windows

Microsoft documents these default folders for current versions of classic Outlook:

Older Outlook versions also kept PST files in the AppData\Local\Microsoft\Outlook folder, and both you and your IT department can place data files elsewhere. The reliable way to find yours is in Outlook itself: go to File → Account Settings → Account Settings and open the Data Files tab. It lists every PST and OST in your profile with its full path. Select one and click Open File Location to jump to it in File Explorer.

AppData is hidden; typing %LOCALAPPDATA%\Microsoft\Outlook into File Explorer’s address bar goes straight there.

OST vs PST at a glance

The size limit is the one thing they share. Our article on Outlook PST size limits covers the registry values that control it.

Why you can’t just copy an OST to a new PC

An OST belongs to the mailbox and Outlook profile that created it. It is a cache, not an archive, and Outlook only uses it to sync with that one account. Copying it to another computer, or adding it to a different profile, won’t give you your mail. Microsoft says plainly that you cannot import the cached messages from an OST that belonged to an Exchange or IMAP account.

The good news is that you rarely need to:

• For Microsoft 365, Exchange, Outlook.com and IMAP, just add the account to Outlook on the new computer. Outlook creates a fresh OST and downloads the mail from the server.
• For mail that isn’t on a server, such as a POP account or anything kept in “On this computer” folders, copy or export the PST files instead. Those are the files that move.

What happens when you delete or rebuild an OST

Rebuilding the OST is a standard fix for sync problems and a damaged cache. For an Exchange account, Microsoft’s instructions are simple: close Outlook and delete the OST. Outlook creates a new one the next time it opens and downloads the mailbox again from the server. (The Inbox Repair Tool, scanpst.exe, is meant for PST files and doesn’t analyze data stored in an Exchange mailbox.)

A safer way to do it:

1. Check that your mail is on the server first, for example in Outlook on the web.
2. Find the OST on the Data Files tab, then close Outlook.
3. Rename the file (for example to name.ost.old) instead of deleting it, so you still have it if something turns out to be missing.
4. Start Outlook and let it download everything. Large mailboxes can take a long time.
5. Once you are happy, delete the old file to free up the disk space.

What you can lose

A rebuilt OST holds only what is on the server. Anything that existed only in the local file is gone with it: messages still waiting in the Outbox, changes that never synced, and, for IMAP accounts, the local contacts and calendar described above. If the server mailbox no longer exists, there is nothing to rebuild from at all.

Getting mail out of an OST-backed account: export to PST

When you need a portable copy of a server-based mailbox, for a backup, an archive or a move, export it to a PST while the account still works. In classic Outlook for Windows:

1. Cache everything first. With Cached Exchange Mode on, Outlook exports only what is in the local cache, which by default is the past 12 months. Set Download email for the past to All (see above), restart Outlook and wait for it to finish syncing.
2. Go to File → Open & Export → Import/Export.
3. Choose Export to a file, then Outlook Data File (.pst).
4. Select the account’s top folder (Outlook exports one account at a time) and keep Include subfolders ticked.
5. Click Browse, choose where to save the PST and give it a name, then click Finish. You can set an optional password.

Save the PST to a local folder, not a OneDrive-synced one, as Microsoft advises. The export doesn’t include rules, folder properties or blocked senders lists.

Microsoft’s documentation also lists a Settings → Files → Export option in the new Outlook for Windows, but PST support there is still limited (it can’t yet show calendar and contacts from a PST), so classic Outlook remains the dependable route. To bring the PST back into Outlook later, see our PST import guide.

Orphaned OST files from deleted accounts

An “orphaned” OST is one whose account no longer exists, or no longer exists in Outlook: a departed employee’s mailbox was deleted, or a Windows reset removed the Outlook profile but left the files behind. Classic Outlook usually warns that removing an account deletes its offline cached content, but OST files from old profiles and installations can still linger on disk.

Outlook can’t open, sync or import an orphaned OST. If the mailbox still exists on the server, ignore the file and let Outlook download a fresh copy in a working profile. If the mailbox is really gone, the OST may be the only surviving copy.

In a business, ask your IT team first: deleted Microsoft 365 mailboxes can sometimes be restored for a limited time, or the mail may be held in a backup or under a retention policy. Failing that, third-party “OST to PST” converters exist that read OST files directly and save the contents as a PST. We don’t recommend specific products. If you try one, work on a copy of the file and check what it actually recovered before relying on it.

MoveMyInbox isn’t one of those tools: it converts MBOX mailboxes (from Gmail Takeout, Thunderbird or Apple Mail) into PST, and it doesn’t read OST files. For a map of all the formats involved, see email file formats explained.

Frequently asked questions