Written by Jennifer Walsh. Reviewed for technical accuracy by Shubham Dixit, Independent Expert in Email Forensics and Data File Conversion.
Shubham is an independent external reviewer and not an employee of PCDOTS.
Quick answer. MBOX is the mailbox format Thunderbird, Apple Mail and many Unix clients use, and Outlook cannot export to it directly. The free route bridges through Thunderbird, which needs both Outlook and Thunderbird installed. A converter reads the PST files directly, with no mail client involved, and handles orphaned or bulk files.
On this page
Why PST and MBOX do not mix?
The reason this needs converting at all comes down to two formats that were never meant to talk to each other. PST is Outlook’s own file, a single proprietary container that holds a whole mailbox, folders, messages, attachments and more, and really only Outlook understands it fully. MBOX is the opposite in spirit, an old and open format where one plain file holds all the messages in a folder, and it is what Thunderbird, Apple Mail and a long list of Unix clients read.
So when you move from Outlook to Thunderbird or Apple Mail, or you just want your mail in a portable format that is not tied to one program, you need to get it out of the PST container and into MBOX. Outlook itself will not do this, it only exports back to PST or to CSV, so the conversion has to happen somewhere else. There are two honest ways to do it, and which one fits depends on what you have installed and how many files you are dealing with.
PST stays inside Outlook. MBOX travels wherever you need it.
“PST is a container, not just a pile of messages, and that is the thing to respect in any conversion. It carries a folder tree, attachments, and metadata, and a good conversion has to carry all of that across, not just dump the message bodies into a flat file. The reason MBOX is worth moving to is that it is open and every serious mail client can read it, so once your mail is there it is no longer hostage to a single application. Just check the folder structure survived, because that is where cheap conversions cut corners.” Shubham Dixit, Email Migration reviewer (draft, pending approval)
Which route fits you?
If Outlook is installed on the machine with your PST open in it, and you do not mind a bit of setup, the free Thunderbird route below will convert your mail for nothing. If your PST files are loose exports with no Outlook to open them, or there are a lot of them, or some are damaged, a converter that reads the PST files directly is the one that will actually cope. Both are covered.
Convert PST to MBOX free with Thunderbird
Thunderbird stores its mail as MBOX natively, so the trick is to get your Outlook mail into Thunderbird and then export it. It is free, it just asks for a little patience.
Step 1. With Outlook installed and your PST open in it, open Thunderbird, go to the menu, choose Tools then Import.
Step 2. Select Mail, then click Next.
Step 3. Choose Outlook and click Next. Thunderbird reads your live Outlook profile and pulls the mail across.
Step 4. Install the free ImportExportTools NG add-on, then right click the imported folder and choose ImportExportTools NG then Export folder, and pick a location. You get a .mbox file you can use anywhere.
The free path bridges through Thunderbird, so both mail clients have to be present.
The catch with the free method
That route works, but be clear about what it quietly assumes, because this is where people get stuck. Thunderbird’s Outlook import reads a live Outlook profile, not a file, so you need Outlook actually installed on the machine with the PST opened inside it. If all you have is a loose PST from an old computer and no Outlook to open it, this method has nowhere to start. It will not touch an OST file either, and it works one profile at a time, so a stack of separate PST files means repeating the whole thing for each. On top of that, the Outlook import can be temperamental with 64 bit Outlook, and importing a large mailbox is slow. None of that makes it a bad option when the pieces line up, it just rarely lines up for bigger or messier jobs.
Convert PST to MBOX with a converter
A dedicated converter solves the free method’s main weakness by reading the PST files directly, so it needs neither Outlook nor Thunderbird installed. That is what makes it the answer for a loose or orphaned PST, for damaged files, and for converting many at once. The PCDOTS PST Converter loads one or many PST files, previews the mail, and writes them out to MBOX in a single run, keeping the folder hierarchy and attachments intact. It can filter by sender, recipient, subject or date so you export only what you need, and it copes with PST files from any version of Outlook. There is a free trial so you can check the result against your own files first.
Converting a stack of PST files to MBOX in one run, no mail client required.
Here are the steps with the screens.
Step 1. Run the converter, click Open, choose Outlook Data Files and load your PST files.
Step 2. Browse, import and preview the mail and folders.
Step 3. Click Export and select MBOX from the format list.
Step 4. Choose a destination and click Save to run the conversion.
Once you have the MBOX files, a free MBOX file viewer lets you open and check them before importing anywhere, and there is a separate guide to export Outlook to MBOX if you are working from a live mailbox rather than PST files.
A note on MBOX and Apple Mail
One thing worth knowing before you hand the file off, because it saves a puzzled half hour later. MBOX is not a single tidy standard, there are a few variants of it, and while Thunderbird and most Unix clients are relaxed about which one they read, Apple Mail can be fussy. Its import expects the mailbox laid out in a particular way, so a raw .mbox that opens fine in Thunderbird sometimes needs a moment of coaxing in Apple Mail through File then Import Mailboxes. If Apple Mail is your destination, test one folder first rather than converting everything and discovering the quirk at the end.
The methods compared
Match the route to what you have and how much of it.
| Method | Best for | Good to know |
|---|---|---|
| Thunderbird bridge | A live mailbox, free | Needs Outlook and Thunderbird both |
| PST converter | Orphaned or bulk PST files | Reads PST directly, keeps structure |
| Free MBOX viewer | Checking the result | Opens MBOX, does not convert |
Frequently asked questions
How do I convert a PST file to MBOX?
Two ways. Free, import the mail into Thunderbird and export the folder to MBOX with the ImportExportTools NG add-on, which needs Outlook and Thunderbird both installed. Or use a converter that reads the PST directly and writes MBOX, with no mail client needed.
Can Outlook export to MBOX directly?
No. Outlook only exports to PST or CSV, so the conversion to MBOX has to happen in another tool, either Thunderbird or a dedicated PST to MBOX converter.
How do I convert a PST to MBOX without Outlook?
The free Thunderbird route cannot help, since it reads a live Outlook profile. Use a converter that opens the PST file directly instead, which works even when Outlook is not installed and on orphaned files.
Will my folders and attachments survive the conversion?
They should. A proper conversion keeps the folder hierarchy and the attachments with each message. It is worth opening the result in an MBOX viewer to confirm the structure came across before you import it.
Why will not Apple Mail open my MBOX file?
MBOX has a few variants and Apple Mail is picky about the layout it expects on import. Use File then Import Mailboxes and choose the mbox option, and test one folder first, since a file that opens in Thunderbird can still need coaxing in Apple Mail.
Can I convert many PST files to MBOX at once?
Yes, with a converter. It loads a folder of PST files and writes MBOX for each in a single run. The Thunderbird route works one profile at a time, so it is slower for a stack of files.
Out of Outlook, into anything
The gap between PST and MBOX is really just the gap between one company’s format and an open one, and crossing it is more straightforward than it looks. If you still have Outlook and Thunderbird on hand, the free bridge will carry your mail across for nothing, patience aside. If you are working from loose PST files, a pile of them, or anything damaged, a converter that reads the files directly will get you there without the mail clients at all. Either way, once your mail is MBOX it belongs to you rather than to Outlook, ready to open in Thunderbird, Apple Mail or whatever you move to next.
