What is the OST file format?
.ost files are used by Microsoft Outlook to store email data locally. The file can contain email messages, tasks, calendar data, contacts, and notes. .ost files serve as an offline mailbox cache when Outlook is not connected to a Microsoft Exchange or Microsoft 365 server.
How .ost files work
.ost is an offline folder file that enables users to perform their usual Outlook tasks without an active internet connection - all changes made offline are synchronized with the Exchange server upon reconnection. This is especially useful in places with limited or unreliable internet access.
Creation and location
An .ost file is created automatically when an Exchange or Microsoft 365 account is first configured in Outlook. The file is stored on the local hard disk drive, not on the mail server. Default path on Windows:
C:\Users\[username]\AppData\Local\Microsoft\Outlook\
The file can be safely deleted; Outlook will rebuild it by re-syncing from the server on next launch.
Similarities to PST format
.ost shares many similarities with the .pst format - both use the same binary PFF on-disk structure and begin with the !BDN file signature (21 42 44 4E hex). However, a .pst is a portable standalone archive, while an .ost is a server-cache tied to a specific mailbox profile and cannot simply be moved to another machine. Since Outlook 2013, IMAP and Microsoft 365 accounts also use .ost rather than .pst files by default.
Security & safety
RISK: MEDIUMAn OST contains a full copy of someone's mailbox - highly sensitive personal/ corporate data - and it's profile-encrypted, so handle it like confidential data and don't upload it to random "free online OST viewer" sites (a common data-leak and scam vector). The file itself isn't executable, but the OST-to-PST tool market is rife with overpriced and fake "free" converters; stick to known vendors (Stellar, Kernel, DataNumen) and prefer the built-in Outlook export when the account still works. Deleting an OST is safe only when the mailbox still exists on the server (Outlook rebuilds it); for an orphaned OST, the .ost may be the only copy.
Format details
in a nutshell- Trillian / iMesh / FrostWire object storage - A few older P2P and messaging apps used .ost for internal object-store data - unrelated to Outlook mailboxes.
- Newsgroup/collaboration offline store (NST) - Outlook's Groups feature uses a closely related .nst offline store; older Exchange/news clients used similar offline tables.
Programs that open OST files
Windows3 apps
Microsoft Outlook (classic)
Paid If the account still works: the OST is used automatically. To make data portable, File → Open & Export → Import/Export → Export to a file → Outlook Data File (.pst). An orphaned OST from another profile cannot be opened directly.
Rebuild the OST (delete + resync)
Built-in If the OST is corrupt: close Outlook, rename/delete account.ost in %LocalAppData%\Microsoft\Outlook, reopen Outlook - it rebuilds the cache from the server. (Only safe when the server still has the data.)
Kernel for OST to PST / DataNumen Exchange Recovery
Paid For orphaned/corrupt OSTs with no working account: these tools parse the .ost and export messages to PST/MSG/EML. Free trials usually preview/limit; full export is paid. Technical details
deep spec| Developer | Microsoft |
| File signature (magic bytes) | `!BDN` (hex `21 42 44 4E`) at byte offset 0, shared with `.pst` |
| MIME type | `application/vnd.ms-outlook` |
| Binary structure | Proprietary PFF (Personal Folder File) binary - same on-disk layout as `.pst`; documented in Microsoft's [MS-PST] open specification |
| Unicode variant identifier | Byte offset 10 = `0x17` marks the modern Unicode (large) OST; `0x0E` identifies the legacy ANSI variant |
| Internal page size | 512 bytes (ANSI/legacy); 4,096 bytes (Unicode/modern, default since Outlook 2003) |
| Default file-size limit | 50 GB in Outlook 2019, 2021 and Microsoft 365; adjustable via `MaxLargeFileSize` registry value |
| Encryption | Compressible encryption enabled by default; no-encryption mode also available; high-encryption (NTB) is not supported for OST |
| Data types stored | Email messages, calendar events, contacts, tasks, notes, and journal entries |
| Default storage path (Windows) | `%LOCALAPPDATA%\Microsoft\Outlook\` |
| Portability | Not portable - tied to a specific mailbox profile GUID; cannot be opened on a different machine without third-party recovery tools |
| Account support | Exchange Server and Microsoft 365 (primary use); IMAP and Outlook.com accounts use `.ost` by default since Outlook 2013 |
| Sync protocol | MAPI over HTTPS via Autodiscover for Exchange/Microsoft 365 accounts |
| Rebuild behavior | File can be safely deleted; Outlook recreates and repopulates it automatically from the server on next launch |
| Released | 1997 (Outlook 97); modern Unicode OST since Outlook 2003 |
| Specification | learn.microsoft.com |
OST conversions
Community Q&A
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