If you've ever tried to drag an MBOX file into Outlook and watched absolutely nothing happen, you already know why this review exists. MBOX and PST are not the same, and Outlook does not support the MBOX file format.
So, when I got stuck moving Thunderbird mailbox archives into Outlook for a client project, I ended up testing not one but both of Stellar Converter for MBOX, the free-to-start online converter and the full desktop software.
In this review, I tested both the online and desktop tools by uploading files, watching progress bars, opening the converted PSTs in Outlook, and checking whether the emails, attachments, and folder structure survived the trip. Here's what I found.
Why You'd Even Need an MBOX to PST Converter in the First Place
MBOX is the file format used by a long list of email clients, including Thunderbird, Apple Mail, Eudora, SeaMonkey, PocoMail, and Google Takeout exports, among others. PST, on the other hand, is Outlook's native storage format. The two aren't compatible, and there's no built-in "Import MBOX" button anywhere in Outlook.
People usually run into this wall for one of a few reasons:
• Switching jobs and moving from Thunderbird or Apple Mail to a company Outlook/Office 365 account
• Migrating an old Gmail account (exported via Google Takeout) into Outlook
• An IT admin consolidating a dozen employees' mailboxes onto one platform
• Just wanting an old mail archive somewhere Outlook can actually read it
The "manual" workarounds IMAP re-syncing, third-party plugins, and copy-pasting individual emails are slow and honestly a little terrifying when you're dealing with years of correspondence you can't afford to lose. That's the gap tools like this are built to fill.
Stellar Online MBOX to PST Converter — My Actual Experience
What it is
This is the browser-based MBOX to PST converter service. No installation, no license key to punch in upfront; you open the page and upload a file.
How it works and what the interface feels like
The layout is refreshingly uncluttered. There's one big upload button, a short "Step 1 → Step 2 → Step 3" explainer above it, and not much else fighting for your attention. I didn't need to hunt for anything or read a manual to figure out where to click, which honestly isn't always true of conversion tools I've used.
Uploading and converting
I dragged in a mid-sized Thunderbird MBOX file first. The upload itself took a minute or so on my connection, and once it landed, the conversion kicked off automatically — no extra settings to fiddle with. A progress indicator shows you roughly where things stand, which is a small thing but it matters when you're staring at a spinning icon wondering if it's actually doing anything.
Speed
For a file under a couple hundred MB, the whole process upload, convert, download wrapped up in under ten minutes. Larger files obviously take longer since you're bottlenecked by your own upload speed as much as by the conversion itself.
Accuracy
I opened the resulting PST in Outlook and went through it folder by folder. Emails, sender/recipient fields, and the original folder hierarchy all came through intact. I did check a few messages with attachments, and those carried over cleanly too.
The genuinely useful bits
• Zero installation works from any browser, including on a Mac or Chromebook where you'd otherwise be stuck
• Preserves the original mailbox folder structure instead of dumping everything into one flat inbox
• HTML preview of the converted file before you commit to downloading it
• SSL-encrypted upload, and Stellar states converted files are auto-deleted from their servers after 24 hours
Where it falls short
Let's be honest about the limitations, because they matter depending on your file size:
• Free conversion is capped at 100 MB per file fine for a light personal archive, not for a decade of work email
• The premium online tier raises the ceiling to 5 GB per file, priced per-file rather than as a flat unlimited license
• You need a working internet connection the entire time, which is a problem if you're on a metered or unreliable connection with a large mailbox
• No batch processing on the free tier: one file, one upload
Who it's actually best for
If you've got a single, reasonably sized MBOX file and you want it converted without installing anything — a one-off Thunderbird export, a small Takeout file — this is genuinely the easier path. I wouldn't reach for it, though, if I were handling a client's 8 GB legacy archive or converting mailboxes for a whole department.
Stellar Converter for MBOX (Desktop) — The Full Review
Getting it installed
The desktop installer is a standard Windows .exe, and setup took maybe two minutes on my machine. It asks for 250 MB of disk space and runs on Windows 8/8.1/10/11 (64-bit). Nothing unusual here no odd permissions requests, no bundled toolbar nonsense.
Which email clients it actually supports
This is where the desktop version pulls ahead noticeably. It reads MBOX files from more than 17 clients, including Thunderbird, Apple Mail, Google Takeout, Entourage, Eudora, PocoMail, SeaMonkey, Opera Mail, Netscape, Spicebird, Evolution, Claws Mail, Mulberry, Sylpheed, Cone, Mutt, and Gnu Mail. If you're dealing with something more obscure than a mainstream client, this is almost certainly the version you want.
Batch conversion
Rather than picking files one at a time, you can point it at an entire folder using the "Select Identity Folder" option, and it'll process every MBOX file inside in one go. For anyone converting mailboxes for multiple users at once, this alone justifies skipping the online service.
The preview pane
Before saving anything, the software shows a three-pane preview that looks a lot like Outlook itself — folder tree on the left, message list in the middle, and the actual email body (plus attachments and header fields) on the right. It's a good way to sanity-check the data before you commit it to a PST, and I appreciated being able to click through individual emails rather than trusting a summary screen.
Search and filtering
There's a built-in "Find File" tool that locates MBOX, MBX, and related files on your system if you're not sure where they're stored, plus a filter option that lets you narrow the conversion by date range or exclude specific sender/recipient addresses. Useful if you only need, say, the last two years of a mailbox rather than the entire history.
Exporting to PST
The core conversion — MBOX to PST — worked as expected. You can create a brand-new PST or append converted mail into an existing one, and the software will auto-split the output if the resulting PST gets too large for comfortable handling.
Office 365 and Live Exchange export
This is a technician-edition feature, and it's a meaningful one if you're doing an actual migration rather than just archiving files. Instead of exporting to a PST and then importing that into Office 365 separately, the Technician version can push mail items straight into an Office 365 mailbox using Microsoft's modern authentication, which cuts out a step and reduces the chance of something getting mangled along the way.
How it handles large files
This is really where the desktop tool separates itself. I ran a considerably larger MBOX file through it than I'd have wanted to upload anywhere, and it chewed through it without needing an internet connection or hitting any size ceiling. Stellar doesn't publish a hard file-size cap for the desktop tool, and in practice it behaves that way; the main constraint is your own hardware, not the software.
Online vs Desktop — Which One Should You Actually Pick?
|
Factor |
Online Converter |
Desktop Software |
|
Installation |
None |
Required |
|
File size limit |
100 MB free / 5 GB paid |
No practical cap |
|
Internet required |
Yes, throughout |
Only to download/install |
|
Batch conversion |
No |
Yes |
|
Supported clients |
17+ |
17+ |
|
Export to Office 365 |
Not available |
Technician edition |
|
Best for |
One-off, small files |
Large mailboxes, IT admins, bulk migrations |
My honest take: if you're converting a single small file and don't want to install software, the online converter does the job and does it quickly. But if you're handling multiple mailboxes, files anywhere near or over a few GB, or you need Office 365 export, the desktop tool is the one that won't leave you stuck halfway through.
Pricing Breakdown
Online converter
• Free — up to 100 MB, single file
• Premium — $19.99 for 1 file up to 5 GB, with bundle pricing available for 50 or 100 file licenses
Desktop software
• Corporate edition — $39, covers MBOX-to-PST conversion across supported clients
• Technician edition — listed at $299, frequently discounted to around $99, which adds Office 365/Exchange export, saving to existing PSTs, and export to RTF/PDF/HTML/MSG formats
Both desktop tiers come with a free trial that lets you preview converted mail before buying, and Stellar backs the desktop purchase with a 30-day money-back guarantee.
Pros and Cons
Pros
• Accurate conversion in both versions — I didn't lose emails, attachments, or folder structure in either test
• Online service needs zero setup and is genuinely fast for small files
• Desktop tool supports an unusually wide range of legacy MBOX clients
• Preview-before-you-save workflow on both, so you're not converting blind
• Technician edition's direct Office 365 export saves a real step for migrations
Cons
• Free online tier's 100 MB cap will feel small if you're dealing with years of mail
• Desktop batch conversion is more of an "admin tool" experience, not something a casual user needs
• Office 365/Exchange export is locked to the pricier Technician tier
• Online converter is a non-starter without a stable internet connection
Final Verdict
Neither version disappointed me on the thing that actually matters, getting MBOX data into a usable, accurate PST file. Where they differ is scope. The online converter is the quicker, no-install option for small, one-off jobs, while the desktop software is the one to reach for if you're dealing with large archives, multiple mailboxes, or need a direct path into Office 365. If I had to pick just one to recommend to most people reading this, it'd be the desktop version — the 100 MB free cap on the online version runs out faster than you'd think once attachments are involved.
FAQs
Is Stellar's MBOX to PST converter safe to use?
Yes. The online service uses SSL encryption during upload and deletes converted files from its servers after 24 hours. The desktop software runs locally on your machine, so your data never leaves your system unless you choose to export it to Office 365.
Can I convert MBOX files for free?
Yes, both versions offer a free option. The online converter is free for files up to 100 MB, and the desktop software has a free trial that lets you preview the converted mail before buying a license to save it.
Which MBOX email clients are supported?
Both versions support MBOX files from 17+ clients, including Thunderbird, Apple Mail, Google Takeout, Entourage, Eudora, PocoMail, SeaMonkey, and others.
Do I need Outlook installed to use the desktop converter?
Yes, you'll need Outlook installed if you want to save the converted mail directly into an Outlook profile. If you're only exporting to a PST file for later use, that's not a requirement.
What's the difference between the Corporate and Technician desktop editions?
The Corporate edition handles standard MBOX-to-PST conversion. The Technician edition adds the ability to save to an existing PST, export directly to Office 365 or Live Exchange, and save converted mail in RTF, PDF, or HTML formats.
Can I convert multiple MBOX files at once?
On the desktop software, yes — using the "Select Identity Folder" option. The free online converter processes one file per upload; the premium online tier supports multi-file licenses.
