How to Convert Outlook PST to MBOX Without Paying for a Converter

You do not have to buy anything. An Outlook PST becomes a standard MBOX file with two free pieces of software — Mozilla Thunderbird and its ImportExportTools NG add-on — in four steps. The paid converters that fill the search results are selling convenience, mostly the ability to do fifty folders in one click. If you have a handful of folders, or one free afternoon, you do not need them.

Here is the free route, including the parts of it that are annoying, and what to do when it does not apply.

Why you cannot just rename a .pst to .mbox

A PST is a database: a B-tree of nodes and blocks holding the message bodies, attachments and folder structure as records, with the raw bytes scrambled by an obfuscation pass Outlook applies by default. An MBOX file is the opposite: plain text, messages one after another, each starting with a line that begins From .

Renaming a PST gives you a binary file with a misleading extension and nothing else. Converting means walking that tree, pulling out each message, and writing it back as ordinary RFC 5322 text. Microsoft published the PST file format specification in 2010, which is why open-source tools can do this at all — but it is real work, not a rename.

One detail worth knowing first: the old ANSI format from Outlook 97–2002 stops working around 2 GB, and archives that hit that ceiling are often the ones people are trying to rescue. The Unicode format that arrived with Outlook 2003 does not have that problem.

The free route: Outlook to Thunderbird to .mbox

This works on Windows, with classic Outlook installed and the PST open in it.

1. In Outlook, confirm the mail is in a PST. If it is still sitting in an account, use File → Open & Export → Import/Export → Export to a file → Outlook Data File (.pst). If you already have the file, open it with File → Open & Export → Open Outlook Data File so Outlook can see it.
2. Install Thunderbird. It is free, and you never have to configure a mail account in it.
3. In Thunderbird, go to Tools → Import, choose to import from another program, and pick Outlook. The wizard copies the mail into Local Folders. This step reads Outlook’s live profile through Windows, not the loose file on disk — which is exactly why the PST has to be open in Outlook on the same machine. If Outlook is not offered as an option, a mismatch between a 64-bit Thunderbird and a 32-bit Outlook installation is the usual reason.
4. Install the free ImportExportTools NG add-on. Right-click any folder in Thunderbird, open the ImportExportTools NG submenu, and choose the option that exports the whole folder (“Export folder”). Pick a destination and you get a standard .mbox file.

That is the entire procedure. Nothing in it costs money.

The tedious part: it is one folder at a time

Export folder does exactly what it says — one folder, one file. Subfolders are separate exports. With six folders that is two minutes; with a corporate archive of sixty nested folders it is genuinely dull, and that tedium is the actual product the paid converters sell.

There is a shortcut once the import has finished. Thunderbird already stores every folder as an MBOX file inside its profile: extensionless files named after the folder, in the Mail/Local Folders directory, each with a .msf companion that is only an index and can be ignored. Copy those files out and add .mbox to the name yourself — they are the same files described in the guide on opening Thunderbird mailboxes on Windows.

If the PST came from a backup and there is no Outlook

The Thunderbird route needs Outlook. When the PST is an orphan — pulled off an old disk, handed over by IT, inherited with a laptop — you have two options that still cost nothing.

The direct one is readpst, from the open-source libpst project: brew install libpst on a Mac, apt install pst-utils on Debian or Ubuntu. Then:

readpst -r -o ~/mail-out archive.pst

That writes one MBOX file per Outlook folder into ~/mail-out, keeping the folder tree. It handles ordinary mail well. Very old archives with messages Outlook stored in its own rich-text form can come out with odd attachments, so check a few before you throw anything away.

The slower option needs no new software: attach the PST to an IMAP account in Outlook, drag the folders into it, then connect Thunderbird to the same account and export from there. It moves your mail through a server, so it is a poor fit for anything confidential.

If what you have is an OLM rather than a PST — the export format of Outlook for Mac — that is a different file entirely, and the step-by-step guide for both formats covers it.

Why an online converter is a bad idea for a real mailbox

Uploading a PST means handing every message, every attachment and every address in it to a company you know nothing about, and taking their word for the deletion. Free tiers also tend to cap uploads at a few hundred megabytes, while real PSTs run to several gigabytes — so the file you most want to convert is the one that will not fit.

For a test file it is fine. For ten years of correspondence, or anything covered by a confidentiality duty, it is not a trade worth making when a local route exists and is free.

Once you have MBOX, stop converting

If your goal was to read the mail, you are done. MBOX is the readable end state, not a step towards another format, and importing it into a second mail client only makes another copy that lives inside that client.

Open the file with a reader instead — that is what Mbox Viewer is for, on Windows and Mac. It opens the file where it sits, never writes to it, and mailboxes up to 1 GB open without paying anything either.