PST to MBOX Converter - Export Outlook Mail to Thunderbird

How to convert PST to MBOX

1. Open the PST or OST file

Point the program at the .pst or .ost file. It is read straight off the disk, nothing else has to be set up first. The folder tree shows up in the left pane, and any message can be opened and read there before you convert anything.

2. Tick the folders you actually need

Select folders in the tree. Empty ones can be skipped, Deleted Items left out, and date range narrows the export to the years that matter. On an old Inbox this is where most of the dead weight falls away.

3. Choose MBOX and the output folder

Set the output format to MBOX on the Convert tab. If a mailbox file with that name is already sitting there, you decide what happens to it. Append is for topping up an archive you converted a month ago. Rename and overwrite are there too.

4. Take the result into your mail client

The output directory is what you hand to Thunderbird. Either import it with the ImportExportTools NG add-on or drop it into the profile folder. Nothing needs renaming or reassembling on the way.

Folder tree in .sbd layout

MBOX carries no hierarchy of its own. To keep Inbox / 2026 / Clients standing, the converter writes every folder as a mailbox file and puts its children into directory named after that file with an .sbd suffix, right next to it. That is the layout Thunderbird itself uses on disk. Most tools in this niche say they preserve the hierarchy and then hand you a flat pile of files to import one by one.

Original internet headers, word for word

For mail that arrived over SMTP, the whole original header block sits inside the PST file and it is copied out verbatim. The Received chain, DKIM signatures, Return-Path and List headers come across as they were written. In a test archive of 18,600 real messages every SMTP-delivered message had that block. Mail you sent yourself never has it, so for those the headers are rebuilt from the MAPI properties instead. Saying 'all headers preserved' without that caveat would simply be untrue.

Real recipient addresses, not display names

To and Cc are assembled from the recipient table of the archive, so the output carries actual SMTP addresses instead of a bare 'Ivan Petrov'. In the same archive that worked for 99.6% of messages, and every recipient found there had an address. Display names look fine in a message list and fall apart the moment somebody hits Reply.

Read status through X-Mozilla-Status

Whether a message was already read is kept inside the PST as a message flag, along with replied and flagged. All three are written into X-Mozilla-Status, hex 0001, 0002 and 0004. That header is the only place Thunderbird looks for those states. Everyone in this category promises read and unread survive. Almost nobody says through what.

Attachments byte for byte

Attachments are written out unchanged, and in our own tests every one of them matched the source by sha256. Inline images stay where they belong in the body, because the cid: reference in the HTML still matches the Content-ID of the part. No re-encoding and no 'attachment converted' stubs.

Filters, streaming and a report

Date range, skip empty folders, exclude Deleted Items, export without attachments, append into an existing mailbox. Writing is streamed rather than buffered, so a heavy Inbox does not need to fit in memory. In our own tests the archive above went through in 49 seconds with no failures, and a plain text report per folder is dropped next to the result.

Where the MBOX output opens

MBOX is a plain concatenation of messages, so one conversion serves several clients. The .sbd directories are a Thunderbird convention. Other clients ignore them and open the mailbox files directly.

Mail client	Platform	How it takes the output
Mozilla Thunderbird	Windows, macOS, Linux	Native on-disk format. Import the output directory with ImportExportTools NG, or copy it into the profile folder.
Apple Mail	macOS	File, Import Mailboxes, Files in mbox format. Imported mail arrives in an Import mailbox and is moved from there.
Postbox	Windows, macOS	Built on the Thunderbird codebase, so it reads the same mailbox files and the same .sbd directories.
Eudora and other legacy Windows clients	Windows	Eudora keeps mail in mbox-style .mbx files, so a mailbox is taken in once it is renamed and placed in Eudora's mail folder.
mutt and other Unix mail clients	Linux, BSD	Output is mboxrd, so a line starting with From in the body is escaped with a leading angle bracket and parsers do not split a message in the wrong place.

Download for Free

PST to MBOX Converter Software

Languages

File Size

4.5 Mb

Version

1.0

Last updated on

12/05/26

$ 19.99

🖥️ System Requirements

Windows 11/10/8.1/8/7 (32/64 bit)
Intel i3, AMD Ryzen 5 or above
4 GB of RAM or above
NVIDIA® GeForce® series 8 and 8M, Intel® HD Graphics 2000, Quadro FX 4800, Quadro FX 5600, AMD Radeon™ R600, Mobility Radeon™ HD 4330, Mobility FirePro™ series, Radeon™ R5 M230 or higher graphics card with up-to-date drivers
1280 × 768 screen resolution, 32-bit color
1 GB of free hard disk space or above

🙋 Frequently Asked Questions

❓ Can I convert a PST file to MBOX without Outlook installed?

Yes. The archive is read directly by the program, so Outlook does not have to be installed. No Exchange profile or mail account either. That is the usual case anyway. People convert PST to MBOX precisely when Outlook is already gone, after an expired subscription or on a machine that never had it.

❓ Can Thunderbird open a PST file on its own?

No. Thunderbird has no PST reader. Its Outlook import menu only works when a configured Outlook sits on the same computer and hands the mail over through MAPI, which is of no use on a machine where Outlook was never installed. Converting the archive to MBOX first removes that dependency.

❓ How do I get the converted mail into Thunderbird?

The output is written in Thunderbird's native on-disk layout, a mailbox file per folder plus a matching .sbd directory for its subfolders, so it is ready for the ImportExportTools NG add-on, whose import list covers mbox files together with their structure. Copying the same directory into the Thunderbird profile folder works too if you prefer to skip add-ons.

❓ Will my Outlook folder tree survive the conversion?

Yes. And it is the one thing worth verifying yourself in any converter. Open the output directory afterwards. Every folder should have its own mailbox file with a matching .sbd directory beside it, and the subfolders should sit inside that directory rather than flattened out into names like Inbox_2026_Clients. That is the shape Thunderbird keeps a nested tree in on disk.

❓ Can messages go missing during conversion?

They should not, and you do not have to take that on faith. Message loss is a real complaint in mail migrations. People compare counts before and after an export and find items missing, usually long after the source archive has been deleted. So every folder is counted here and the counts go into plain text report next to the result. Compare source and output folder by folder instead of trusting a progress bar.

❓ Are read, replied and flagged marks preserved?

In Thunderbird, yes. The state rides in the X-Mozilla-Status header and Thunderbird picks it up on import, so a converted mailbox arrives with the same messages already marked as before. Other mail clients treat that header their own way and most simply ignore it, so the same mailbox can show up as all unread there.

❓ Is there a free way to convert PST to MBOX?

There are free routes and they are worth knowing before you pay for anything. If Outlook and Thunderbird are both installed and working on the same PC, Thunderbird can import from Outlook directly. Open source readers pull the messages out of a PST file on command line. What those routes normally leave you without is the tree in the layout Thunderbird wants, and there is no date filter to keep the batch small. For one small archive, free is probably enough. It stops being enough at the point where the tree and the read marks have to be right on the first pass.

❓ Which mail clients can open the MBOX files it writes?

Thunderbird first of all. Postbox shares its codebase, so it reads the very same files. Apple Mail imports them on macOS, and on Linux mutt reads them as they are while Claws Mail imports them. The files are mboxrd, so a From line inside a message body is escaped and no parser splits the mailbox in the wrong place. The table above has the rest.

❓ How do I export my mail from Outlook to a PST file first?

In classic Outlook the path is File > Open and Export > Import/Export > Export to a file > Outlook Data File (.pst). Pick the top of the mailbox and tick Include subfolders so that the whole tree goes in. If your mail sits in an .ost cache instead, there is nothing to export, because .ost files open here directly.

❓ Can it convert several PST files in one run, or repair a damaged or password-protected archive?

One archive per session for now. A queue of several files is planned but not in this version, and we prefer to say so up front. Repairing a corrupted PST is a different class of tool and is not attempted here: if the file does not open, you get an honest error rather than a half-converted mailbox. A PST locked with a password is handled by our separate Outlook PST Password Recovery product, not by this one.

PST to MBOX Converter for Windows