How to Open Outlook PST and OST Files Without Outlook

Sooner or later someone hands you a .pst file. Maybe it's an old mailbox export from a former employee, an archive from a laptop you retired years ago, or evidence you need to review. You don't have Outlook installed, you're on Linux or macOS, or you simply don't want to set up a mail profile just to read a few messages.

This post covers what these files actually are and the practical ways to read them without Outlook, along with the privacy trade-offs of each approach.

What PST and OST files are

PST (Personal Storage Table) is Microsoft's file format for storing mail, contacts, calendar items and tasks locally. Outlook uses it for archives and for POP3 accounts, and it's what you get when you use File → Import/Export → Export to a file.

OST (Offline Storage Table) is a local cache of a server mailbox (Exchange, Microsoft 365, and IMAP accounts in newer Outlook versions). It lets Outlook work offline and syncs back to the server.

A few useful facts:

• Both formats share the same underlying structure, documented by Microsoft as [MS-PST]. Internally it's a set of B-trees (a node B-tree and a block B-tree) with a "messaging layer" on top that represents folders, messages, and attachments as property sets.
• There are two main variants: the old ANSI format (Outlook 97–2002, 2 GB limit) and the Unicode format (Outlook 2003 and later), which supports much larger files. Multi-gigabyte PSTs are common.
• PSTs can be password-protected, but the "password" is a weak check stored in the file, and the content is usually only obfuscated with a simple encoding, not strongly encrypted. Don't treat it as a security boundary.
• An OST is tied to the profile it was created for. Outlook itself won't open someone else's OST as a data file, but third-party parsers can often read it because the on-disk structure is the same.

Because the format is documented, there are good open-source parsers. You have real options.

Option 1: Use Outlook anyway (if you have access)

If you have a Windows machine with Outlook, it's still the most faithful viewer:

1. File → Open & Export → Open Outlook Data File to mount a PST alongside your mailbox, or
2. File → Open & Export → Import/Export → Import from another program or file to merge it.

This doesn't work for OST files from another profile. And it requires a licensed Outlook install, which is exactly what many people lack.

Option 2: libpff / pffexport (Linux, macOS)

libpff by Joachim Metz is a forensic-grade library for PST, OST, and PAB files. It ships with command-line tools:

• pffinfo shows file metadata (format type, encryption type).
• pffexport extracts the contents into a directory tree.

On Debian/Ubuntu:

sudo apt install pff-tools
pffinfo mailbox.pst
pffexport -m all -t out mailbox.pst

You get folders containing one directory per message, with headers, the body (plain text, HTML, or RTF), and attachments as separate files. It's great for forensic work or for grepping through mail, but it's not a mail client. libpff also handles OST files and can recover deleted items in some cases (-m all includes recovered and orphaned items).

Option 3: readpst → MBOX → Thunderbird

readpst comes from the libpst project. Its main advantage is that it converts to standard formats that ordinary mail clients understand.

# Debian/Ubuntu
sudo apt install pst-utils
# macOS
brew install libpst

mkdir out
readpst -r -o out mailbox.pst      # one MBOX per folder, preserving hierarchy
readpst -e -o out mailbox.pst      # one .eml file per message instead

Then, to read the result comfortably:

1. Install Thunderbird and the ImportExportTools NG add-on.
2. Create a Local Folder, right-click → ImportExportTools NG → Import mbox file.
3. Choose the MBOX files readpst produced (the "with sub-folders" option keeps the structure).

Now you have full search, threading, and attachment handling, all offline. This is my default recommendation for anyone comfortable with a terminal: everything stays on your machine, the tools are free and open source, and MBOX/EML are open formats you can keep long-term.

Caveats: very large or damaged files sometimes trip up readpst. If that happens, try pffexport, which is often more tolerant. Calendar and contact items are exported too, but they're less pleasant to browse than mail.

Option 4: Browser-based viewers

There's a class of tools that let you open a PST in a web page. They differ a lot in where the parsing happens, and that's the part you should check.

Upload-based converters send your file to a server, process it there, and give you back a download. They're convenient, but think about what's in a typical mailbox: contracts, password-reset emails, HR conversations, customers' personal data. Uploading that to an unknown third party may violate your company's policy or data-protection obligations (GDPR, client confidentiality, legal hold). Also, multi-GB uploads are slow and often fail.

Client-side viewers parse the file in the browser using JavaScript or WebAssembly. The file never leaves your machine. You can verify this yourself: open DevTools → Network, load a file, and watch for requests carrying its contents. Or disconnect from the network after the page loads and check that it still works.

One option in this category is Mailward (https://pst.aivismonitor.com/). Disclosure: I built Mailward. It parses PST/OST files locally in a Web Worker with no upload, handles multi-GB files, and shows the folder tree, message bodies, and attachments, with full-text search. Reading and searching are free (with 3 exports per day), and a $19 one-time Pro license adds batch MBOX/EML export. If you're happy in a terminal, readpst + Thunderbird above does a similar job for free.

Whatever browser tool you pick, run the DevTools check before you drop in a sensitive mailbox.

Step-by-step: which path should you take?

1. Identify the file. Run pffinfo file.pst (or check the extension and size). Note whether it's PST or OST, ANSI or Unicode.
2. Make a copy. Always work on a copy, especially with evidence or with a file that might be damaged. Some tools open files read-write.
3. Decide where the data is allowed to go. If the mailbox contains anything sensitive, rule out upload-based services. Stick to local tools (libpff, readpst, a client-side viewer) or Outlook.
4. Pick a tool for the job:
   • Need to look up a few emails quickly: a local browser viewer or pffexport + grep.
   • Need a long-term, searchable archive: readpst → MBOX → Thunderbird.
   • Need forensic completeness or deleted items: pffexport -m all.
   • Have Outlook and a PST from your own profile: just open it in Outlook.
5. Verify the result. Compare folder counts and spot-check a few messages with attachments and non-ASCII text (encoding issues show up there first).
6. Clean up. Exported folders contain plaintext copies of everything. Delete them securely when you're done, or store them with the same care as the original.

Wrapping up

PST and OST look like proprietary black boxes, but the format is documented and well supported by open-source tools. On Linux and macOS, readpst and pffexport cover most needs. Thunderbird gives you a comfortable reading experience, and client-side browser viewers are a zero-install option. Whatever you choose, think about where your data goes before you think about convenience.