What Is an OST File? Outlook's Offline Cache Explained

An OST file is the local cached copy of a server mailbox that Outlook keeps on the user's hard drive. The acronym stands for Offline Storage Table. When a laptop is on the road and the user opens Outlook, the messages they see are coming out of the OST, not out of Exchange or Microsoft 365. Once the connection is back, Outlook reconciles the two. This entry covers what OSTs are, how they differ from PST files, why they are useless when copied between machines, and what to do when you find an orphaned one during a migration.

The short definition#

OST is a proprietary Microsoft file format used by Outlook in cached Exchange mode. It mirrors the contents of an Exchange, Microsoft 365, or modern IMAP account locally so Outlook can search, read, draft, and queue mail without an active server connection. When the connection returns, the OST syncs back up.

On disk, an OST is a single binary file with the .ost extension. Sizes range from a few hundred megabytes to over 50 GB for users with large mailboxes and "keep everything offline" enabled.

OST vs PST — the difference that matters#

PST and OST share a lot of internal structure but serve completely different jobs.

• A PST is a primary store. It is authoritative. If you delete the PST, you lose data.
• An OST is a cache. The server is authoritative. If you delete the OST, Outlook just downloads a fresh copy on next sign-in.

This single distinction drives almost every operational decision about the two formats. PSTs get backed up. OSTs get rebuilt.

Why an OST locks to one profile#

When Outlook creates an OST, it embeds metadata identifying the Outlook profile and the mailbox GUID. If you copy john@example.com.ost from one machine to another and try to open it, Outlook compares the metadata to the current profile, finds a mismatch, and refuses to load it. The file is still there, the data is still inside, but Outlook will not let you in.

This is intentional. It prevents accidental data leakage when a laptop is reassigned. It is also the thing that makes OST recovery during migrations awkward.

Recovering data from an orphaned OST#

You will run into orphaned OSTs in a handful of common scenarios:

• A user leaves and their Microsoft 365 mailbox is deleted, but their laptop is still in the office with an OST full of mail.
• A tenant migration finishes and the old mailbox is decommissioned, but you discover later that some folders were never moved.
• A profile gets corrupted and the user cannot re-attach to the server.

Three options, in order of preference:

1. Re-attach the OST to a live mailbox. If the original mailbox still exists, recreate the profile on the same machine. Outlook will resync the OST against the server and you can export from there.
2. Open the laptop in offline mode and use Outlook's File → Open & Export → Import/Export wizard to write the OST contents out to a PST while the account is still configured.
3. Use a recovery tool that reads OST directly. Several commercial tools convert OST to PST or EML by parsing the binary format. This is the path you take when the original mailbox is gone and the profile is broken.

Once the data is in a PST or EML or MBOX, it becomes much easier to handle — see the PST, MBOX, EML migration guide for the next steps.

OST size limits and corruption#

The modern OST format (Unicode) has a default 50 GB ceiling, configurable via a registry setting up to about 100 GB. Approaching the limit causes performance problems long before Outlook actually refuses writes — searches slow to a crawl, sync stalls, and the file becomes corruption-prone.

Outlook ships with scanpst.exe, but it only handles PSTs. For OSTs the equivalent is to delete the file and let Outlook rebuild it from the server. Provided the server still has the mail, this is fast and safe. If the server data is already gone, deletion is destructive — recover first.

OST in a migration plan#

For most PST to Microsoft 365 migrations, OSTs are not in scope. The server is the source of truth and you migrate from the server. Where OSTs become relevant is the cleanup pass — discovering laptops with cached mail that was never properly archived. Walk through the user laptops at the end of a migration, export each OST to PST while the old profile still works, and store the results centrally before reimaging.