PST & OST Viewer: Open Outlook Data Files Without Outlook

About the PST & OST Viewer: Open Outlook Data Files Without Outlook

An Outlook data file holds a whole mailbox in one file. POP accounts keep everything in a .pst file, archives are .pst files too, and Outlook.com, IMAP, Exchange and Microsoft 365 accounts keep a copy of the mailbox in an .ost file (Microsoft). This page opens either one in your browser, with no Outlook: the folder tree, each folder's mail, contacts, calendar items, tasks and notes, every item with its attachments, and a search across the whole file.

The reader is built from Microsoft's own description of the format, [MS-PST], which gives "the necessary technical information required to read and write" these files (Microsoft Open Specifications), and it also reads the variant with 4 KB pages that offline files of recent Outlook versions use. It reads only the parts it needs, a few small pieces at a time, so even a file of tens of gigabytes never has to fit in your device's memory. Nothing is uploaded, and the file is never changed.

Common uses

• Reading an old archive (.pst) on a computer without Outlook, on a Mac, or on a Chromebook.
• Finding one e-mail, attachment or contact in a large data file.
• Checking what the .ost file of a closed account still holds.
• Looking through a data file someone gave you before you import it into Outlook.
• Saving the attachments of a few messages without setting up Outlook.

PST or OST: which file you have

• .pst (Outlook Data File): the whole mailbox of a POP account, and the archives and backups people make themselves. It can be copied to another computer and opened there (Microsoft).
• .ost (Offline Outlook Data File): the copy Outlook keeps of an Outlook.com, IMAP, Exchange or Microsoft 365 mailbox (Microsoft). Outlook makes a new one whenever the account is added again, and Microsoft notes that it can't be moved to another computer (Microsoft), so an .ost is not usually a backup. When the mailbox is gone from the server, as with an account that was closed, the .ost may hold the only copy left.

Files in the older ANSI format stop at about 2 GB; the Unicode format of current files can grow to 50 GB by default (Microsoft). The viewer reads both, and its toolbar says which one a file is.

Opening a .pst without classic Outlook

New Outlook for Windows can open, read and search .pst files, but only with classic Outlook also installed, both in the same 32-bit or 64-bit build, and an account with a Microsoft 365 subscription, and the ARM version of new Outlook does not open them (Microsoft). This page needs none of that: it runs in a current browser on Windows, macOS, Linux or ChromeOS, and installs nothing. Classic Outlook itself opens .pst files, when you want the mail back in Outlook.

Passwords, encryption and other people’s mail

• A .pst password is a check that every program must make before it shows the file (MS-PST). This page makes it: the file opens only after its own password is typed, and the page never tries to guess one.
• Messages encrypted with S/MIME open only with the recipient's certificate and private key, so they stay unreadable here; signed messages are shown, without checking the signature (MS-OXOSMIME).
• A data file encrypted with Windows Information Protection is encrypted for the organisation that set it up (MS-PST), and this page cannot read it.
• Open only mailboxes you own or may examine. The .ost of a work account is the organisation's data.

How the reader works

Every item in an Outlook data file is a set of properties (subject, sender, dates, body, attachments) stored in pages and blocks that the file indexes in two B-trees (MS-PST). The reader walks those indexes from the file's header, undoes the simple encoding most .pst files use (it needs no key), and reads each folder's table of items. Bodies come as plain text, HTML or compressed RTF (MS-OXRTFCP); HTML is shown as text, never as a web page, so nothing in a message can run or load anything. Offline files of recent Outlook versions use 4 KB pages and compressed blocks, which [MS-PST] does not describe; the reader handles them, and was checked against offline files written by Outlook.

Every page and block is checked against its checksum. A file with a few damaged parts still opens, with a warning; one that does not open at all can go to the converter's repair mode.

Trademarks

Microsoft, Outlook, Windows and Exchange are trademarks of the Microsoft group of companies; Mac and macOS are trademarks of Apple Inc., registered in the U.S. and other countries and regions; Chromebook and ChromeOS are trademarks of Google LLC; Linux is the registered trademark of Linus Torvalds in the U.S. and other countries. They are named only to say which files and computers the page works with. MySmartCoPilot is not affiliated with Microsoft, Apple or Google, and none of them has sponsored or approved this page.

Limitations

• Messages are shown as text: the layout and pictures of an HTML message are not drawn. Its pictures are listed with its attachments, and the converter’s EML export keeps the HTML as it was.
• The page only reads. It never changes the file, and it cannot move, delete or add items or make a new .pst.
• S/MIME-encrypted messages stay unreadable without the recipient’s certificate, and files encrypted with Windows Information Protection cannot be read.
• A badly damaged file may not open. Outlook’s own Inbox Repair tool (SCANPST.EXE, in Outlook’s program folder) repairs .pst files for free (Microsoft), and the PST & OST Converter’s repair mode reads what is left of a damaged .pst or .ost.
• Very large folders take longer to list, and a search of every item’s text opens each item in turn, which takes longest on large files.
• Archives of Outlook for Mac (.olm), single saved messages (.msg, .eml) and MBOX files are other formats. For .msg and .eml files, use Email to PDF.
• A file that Outlook has open, or that is still being copied or synced, may not read to the end: close Outlook and choose the file again.

Privacy

On this device: nothing is sent

The data file is read by your browser on your device: nothing from it is uploaded or stored, and the page works offline once it has loaded. Use it only on mailboxes you own or may examine; the .ost of a work account belongs to the organisation.