Open an OST file — no Exchange connection needed
Drop an Outlook .ost cache file and browse, read, and search it right here. The file is parsed on your own machine with no server connection and no Outlook — it never leaves your device.
No archive at hand? — 12 messages, 5 attachments, all invented. What is in it
Can I open an OST file without the original Outlook account?
Yes — but not with Outlook. An OST uses the same [MS-PST] container format as a PST, so the file itself is perfectly readable; the lock is that Outlook treats an OST as the offline cache of one specific mailbox and refuses to open it without the matching profile and account. Renaming .ost to .pst does not help, because Outlook inspects the file, not the extension. A reader that parses the container directly — the way Mailward does, locally in your browser with nothing uploaded — never consults the account association, so the folder tree, messages, and attachments in an orphaned OST stay accessible, and can be exported to open formats like MBOX or EML. Mailward reads every header version of the container, including the 4 KB-page layout (version 36) that Outlook 2013 and later use for OST caches; the only files it refuses are those marked with Outlook's 'high' encryption. That is a capability, not a permission: only open OST files whose mailbox data you have the right to access.
Drop the .ost file — it is read locally, nothing is uploaded.
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Open an OST file — no Exchange connection needed
An OST is the offline cache Outlook keeps for an Exchange or Microsoft 365 mailbox, and Outlook refuses to open one unless it can verify the original account — which makes a perfectly healthy backup unreadable the moment the account is gone. Mailward reads the OST format directly, the same way it reads PST, so mailbox data you are entitled to stays accessible: folder tree, messages, attachments, and full-text search, all parsed locally in a Web Worker.
Reading is free, and so is exporting three messages a day; batch conversion is the paid step. Make sure you have the right to access the mailbox data before opening a file that is not yours.
Why Outlook locks you out of your own OST
OST shares much of PST's B-tree container and property model. Microsoft's [MS-PST] specification describes PST; reverse-engineered references such as libpff document OST-specific differences. A compatible direct reader can access cached data without Outlook, but the cache may hold only a date range or header-only items, not the full server mailbox.
The lock is not in the file, it is in Outlook. An OST is the offline cache for one specific mailbox, and Outlook will only open it in the context of the profile and account it belongs to. Remove the account, lose the tenant, leave the company, and Outlook declines to open a file whose contents are sitting right there on your disk. Renaming .ost to .pst does not work either — Outlook checks the file, not the extension.
Because Mailward reads the container format directly rather than asking Outlook to do it, no account association is consulted. That is a capability, not a permission: it is on you to have the right to the mailbox data in a file before you open it. Only open OST files you are entitled to read.
What the file itself declares, before any account check: the first four bytes of every PST and OST are the signature !BDN, and byte 10 is the format version — 14 or 15 for the ANSI layout (Outlook 97–2002), 23 for Unicode (Outlook 2003 onward), and 36 for the Unicode layout with 4 KB pages that Outlook 2013 and later write for OST caches. Mailward's engine reads all three; the 4 KB-page layout is the one that matters for OST, because a reader that only implements the 512-byte page size stops at that byte with an unrecognised-version error. The extension is never consulted in this check, which is also why renaming to .pst changes nothing for Outlook. One version Mailward does refuse on purpose: 37, which [MS-PST] reserves for files protected by Windows Information Protection.
Encryption is the other header-level gate, at byte 461 (ANSI) or 513 (Unicode): the bCryptMethod value. 0x00 is plain storage; 0x01 is the 'compressible encryption' Outlook applies by default, a fixed byte-substitution table that any reader undoes in place as it goes, so it protects nothing; 0x02 is the 'high' (cyclic) option. Mailward decodes 0x00 and 0x01 transparently and stops at the header with an error for 0x02 rather than producing garbage. None of this involves a password — the Outlook 'PST password' is stored as a checksum in the message store and never encrypts the data, so it does not stop a direct reader either.
- Underlying format
- PST-related container; OST differences documented by libpff
- File signature
- Bytes 0–3 are !BDN; bytes 8–9 are SO (53 4F) in an OST, SM (53 4D) in a PST; bytes 10–11 are the version, little-endian: 0E/0F 00 (14/15) ANSI, 17 00 (23) Unicode, 24 00 (36) Unicode with 4 KB pages
- Outlook 2013+ OST
- Version 36 (4 KB pages) — read by Mailward; version 37 (Windows Information Protection) is refused
- Encryption byte
- bCryptMethod 0x00 none / 0x01 compressible (decoded on the fly) / 0x02 high — not supported, open fails at the header
- Why Outlook refuses
- Bound to the original profile/account, not to the file
- Renaming to .pst
- Does not work — Outlook inspects the file itself
- Needs Exchange or M365?
- No — read entirely offline, on your machine
- Your responsibility
- Only open mailbox data you have the right to access
Comparing formats? PST, OST, OLM, MBOX & EML explained — what each one is internally, and what is lost converting between them.
Before you trust a mail file to a website
Q. Do I need the original Exchange account?
No. Mailward reads the OST file directly on your device — no server connection, no account verification, no Outlook. Only open files whose mailbox data you have the right to access.
Q. Is an OST different from a PST?
They share the same container format. A PST is a standalone archive you export on purpose; an OST is the offline cache Outlook maintains for a connected account. The one structural difference you may meet: OST caches written by Outlook 2013 and later use a 4 KB-page layout (header version 36) that older PST-only readers reject. Mailward reads both layouts.
Q. Will my mailbox be uploaded anywhere?
No. The file is read with the browser's File API and parsed in a Web Worker on your machine. You can disconnect from the network after the page loads and keep reading.
Q. Can I turn the OST into something other apps read?
Yes — with a license you can batch-convert folders to MBOX or EML, the open formats Thunderbird, Apple Mail, and most clients import. Single-message EML export is free for three messages a day.
Q. What programs can open an OST file without Outlook?
Four kinds of tool read the container directly. Xst Reader is a free, open-source Windows viewer whose README says reading .ost files was the reason it was written; it needs only .NET Framework 4, and its XstPortableExport command-line companion also runs on macOS and Linux. libpff's pffexport is a cross-platform command-line extractor that lists OST among its supported types, including the 4 KB-page compressed OST layout. The Windows 'OST viewer' downloads that fill search results (Kernel, SysInfo, EdbMails, Stellar and others) preview an orphaned OST free and charge for exporting it. Mailward is the browser option: nothing to install, works on macOS, ChromeOS, Android and iPhone as well as Windows, and the file is parsed on your own device.
Q. How do I open Outlook files without Outlook?
Go by the extension, because Outlook writes four different mail files and each is its own container. A .pst (an archive or export) and a .ost (the offline cache of a connected mailbox) share the [MS-PST] format, and both begin with the bytes !BDN. A .msg is one message saved or dragged out of Outlook on Windows, stored as a Compound File Binary document that begins D0 CF 11 E0. An .olm is what Outlook for Mac exports, and it is a ZIP archive. Mailward opens all four in the browser — the PST viewer on the home page, this page for OST, and the MSG and OLM viewers — with no Outlook, no account and no upload, and it checks those first bytes rather than trusting the name. One Outlook file it cannot open is winmail.dat: that is not a mailbox but the TNEF wrapper around a single Rich Text message.
Q. Why can't Outlook open my .ost file?
Because Outlook treats an OST as a disposable cache, not as a document. It only opens an OST through the mail profile and account that created it, and there is no 'open this .ost' command the way there is for a .pst; Microsoft's own support article on the file says it is a copy of your Exchange information that you can recreate at any time. Once the account is removed, the password changed, or the tenant gone, Outlook has no profile to attach the cache to and refuses it — even though every message is still sitting in the file. Renaming it to .pst does not help, because Outlook checks the header, not the extension. A direct reader like Mailward skips the profile check and reads the messages that are there.
Q. How do I convert OST to PST without Outlook?
Mailward does not write PST — no browser tool does, because producing a valid [MS-PST] file means building its B-trees and property tables from scratch. Getting mail from an OST into a new PST needs a writer of that container: Outlook itself (File → Open & Export → Import/Export → Export to a file, which only works while the account's profile still opens the OST) or a commercial OST-to-PST converter. If the real goal is to read the mail in another client, the shorter route is OST → MBOX or EML: Thunderbird, Apple Mail and most other clients import those directly, and Gmail accepts them through an IMAP client. Mailward previews the OST free and converts folders or the whole cache to MBOX or EML with a one-time license.
Q. Does it open an OST from a Microsoft 365 or Outlook 365 mailbox?
Yes. Outlook 2013 and later cache Microsoft 365 and Exchange mailboxes in the 4 KB-page layout — header version 36 — which is the one variant many PST-only readers reject with an unrecognised-version error. Mailward's header preflight accepts version 36 alongside the 512-byte-page Unicode (23) and ANSI (14/15) layouts. The only OSTs it refuses on purpose are those marked with Outlook's 'high' encryption (bCryptMethod 0x02) and version 37, which [MS-PST] reserves for Windows Information Protection.
Free viewing, search, and 3 single-message exports a day. MBOX reading is free through the first 500 MB; listing and search cover the whole file. Batch conversion is a one-time $19 licence — no subscription, no recurring fees.
Other formats & archives
Same local-first engine, every common mail format.