OST File Documentation

---

Technical details

Feature	Value
Full name	Outlook Offline Storage Table (Offline Outlook Data File)
File extension	.ost
MIME type	application/vnd.ms-outlook
Format type	Offline mailbox cache database (binary)
Developer	Microsoft
Underlying format	PFF / [MS-PST] (same family as PST)
Introduced	Outlook 97; modern Unicode OST since Outlook 2003
Open standard	No — but the PST/OST format is published as [MS-PST]
Role	Cache of a server mailbox (Exchange / Microsoft 365 / IMAP)
Authoritative copy	On the server; the OST is a synchronized local copy
Profile binding	Tied to one Outlook profile and Windows user; profile-encrypted
Byte order	Little-endian
Magic number	21 42 44 4E (“!BDN”) at offset 0
Default location	%LocalAppData%\Microsoft\Outlook
Typical name	account@domain.com.ost
Related extensions	.pst, .msg, .nst, .eml
Specification	learn.microsoft.com/openspecs/office_file_formats/ms-pst/

What is an OST file?

OST stands for Offline Storage Table (Microsoft also calls it the Offline Outlook Data File). It is the local cache that Microsoft Outlook creates when an account runs in Cached Exchange Mode, used with Microsoft Exchange, Microsoft 365 / Outlook.com, and IMAP accounts. The .ost holds a synchronized copy of the mailbox — emails, calendar, contacts and tasks — on the PC, so Outlook stays fast and usable with no network connection. Changes you make offline are queued and sync back to the server when the connection returns.

The defining fact about an OST is that it is a cache, not the primary copy. The authoritative mailbox lives on the server; the OST is a local mirror that Outlook can rebuild from scratch. That is why you do not, and mostly cannot, open an .ost directly the way you open a document. Files live in %LocalAppData%\Microsoft\Outlook and are named after the account, for example yourname@company.com.ost. People run into a standalone .ost when a profile breaks, a PC dies, an employee leaves, or a mailbox is deleted, leaving an orphaned cache as the only remaining copy of the mail.

Built on [MS-PST]: the same format as a PST

Internally an OST uses the identical on-disk format as a PST, the structure Microsoft documents publicly as [MS-PST] (the Personal Folders File, or PFF). Both files begin with the same 4-byte signature !BDN (21 42 44 4E), and both are organised as a layered database rather than a flat list of messages.

OST / PST file structure ([MS-PST])
  header            "!BDN" signature, CRCs, version, encryption type, root pointers
  NDB layer         Node Database: pages and blocks — the low-level storage
   └─ nodes/blocks  B-tree of nodes; each node holds a piece of the data
  LTP layer         Lists, Tables, Properties built on top of the NDB
  Messaging layer   folder hierarchy, message objects, attachments, named properties
  (OST only) sync state / change keys — tracks what has and hasn't synced
  (OST only) profile binding — ties the cache to one Outlook profile

The header records the format version. A byte at offset 0x0A distinguishes the old ANSI format (value 0x0E/0x0F, limited to 2 GB) from the modern Unicode format (0x17), which lifted the size limit and is standard since Outlook 2003. On top of the raw node/block database, a Lists-Tables-Properties layer builds the tables and property sets that the messaging layer turns into folders, messages and attachments. This shared structure is exactly why an OST can be converted into a PST: the underlying object model is the same.

Profile binding: why an OST won’t open on another PC

Despite sharing PST’s format, an OST behaves differently in two ways that matter. First, it is tied to a single Outlook profile and Windows user through the account’s MAPI entry. Second, its data is obfuscated with profile-specific information. Together these mean you cannot copy an .ost to another machine, or add it to a different profile, and expect Outlook to open it — Outlook rejects an OST that does not match the current profile. This binding is deliberate: the cache is meant to be disposable and re-creatable from the server, not portable.

The practical fallout is the entire “how to open an orphaned OST” problem. When the account still exists, the OST is not a portability tool at all; you re-add the account on the new PC and Outlook builds a fresh cache. When the account is gone — deleted mailbox, wiped Exchange server, decommissioned profile — the orphaned .ost may hold the only copy of the mail, and its profile binding is now the obstacle to reading it.

Getting data out: export, rebuild, or recover

There are three distinct situations, and the right action depends on whether the account still works.

Account still works        →  File → Open & Export → Import/Export → Export to a
                              file → Outlook Data File (.pst).  FREE, supported.
OST is corrupt (server OK) →  Close Outlook, rename/delete the .ost in
                              %LocalAppData%\Microsoft\Outlook, reopen → it
                              rebuilds the cache from the server. FREE.
Orphaned OST (no account)  →  A third-party OST-to-PST recovery tool parses the
                              binary directly and exports to PST / MSG / EML.

The supported, free route is always to export from a live account: Outlook writes a portable PST you can copy and open anywhere. If the OST is merely corrupt and the server still has the data, deleting it forces a clean resync — safe precisely because the OST is only a cache. Only when the account is truly gone do you need a recovery tool (Stellar, Kernel, DataNumen and similar), which reads the !BDN structure directly and extracts messages to PST, MSG or EML. There is no free Microsoft utility that converts an orphaned OST; the tool market is paid, and the free-trial versions typically only preview.

Privacy: an OST is a full copy of a mailbox

An OST is not dangerous to open in the malware sense — it is passive data with no executable code. The real concern is confidentiality. The file contains a complete local copy of someone’s mailbox: every email, attachment, calendar entry and contact that had synced. That is highly sensitive personal or corporate data sitting in a single file on disk.

Two practical cautions follow. Do not upload an .ost to a random “free online OST viewer” site to read it — that hands a full mailbox to an unknown third party and is a common data-leak and scam vector; use a reputable local tool instead. And do not delete an orphaned OST assuming it will come back: deletion is safe only when the mailbox still exists on the server (Outlook rebuilds it), whereas an orphaned OST may be the last remaining copy. The OST-to-PST converter market is also full of overpriced and fake “free” utilities, so stick to known vendors and prefer the built-in Outlook export whenever the account still works.

Frequently asked questions

What is the difference between OST and PST?

Both use the same [MS-PST] format, but an OST is an offline cache of a server mailbox (Exchange, Microsoft 365 or IMAP), tied to one profile and rebuildable from the server. A PST is a portable, standalone store you can copy and open on any Outlook. PST is what you export for backup or migration.

Can I open an OST file without Outlook?

Not directly — an OST is profile-bound and obfuscated. Use an OST-to-PST or OST-to-EML recovery tool to extract the messages, then read them in any mail client. Avoid “online OST viewer” sites, which would receive a full copy of confidential mail.

Can I delete my OST file?

Yes, if the mailbox still exists on the server: close Outlook, delete account.ost, and Outlook rebuilds the cache on next launch. Do not delete it if it is an orphaned OST from a deleted or disconnected account, because it may be your only copy of the mail.

References

• Microsoft — Introduction to Outlook Data Files (.pst and .ost)
• Microsoft — [MS-PST]: Outlook Personal Folders (PFF) File Format
• Microsoft — Export or back up email, contacts and calendar to a PST

Feedback

Was this page helpful?

Yes No

Glad to hear it! Please tell us how we can improve.

Sorry to hear that. Please tell us how we can improve.