Unlike a PDF or Word document, an MBOX file isn’t something you can easily open and browse. You normally need a suitable email application or a dedicated viewer to read the messages inside.

One option is MBOX Viewer by Read Mbox, a browser-based tool designed to make opening and working with email archives simpler.

You can try MBOX Viewer for yourself. I’ve tested both the free version and Pro, and the findings below reflect my own experience.

Disclosure: The developer provided me with a free Pro licence so I could test MBOX Viewer. This review reflects my own testing and opinions.

What is an MBOX file?

An MBOX file is a way of storing multiple email messages together in a single file. Think of it as a container holding a collection of emails, often including their attachments.

You might come across an MBOX file when:

• Downloading your emails: Services such as Google Takeout allow you to download a copy of your Gmail messages as an MBOX file.

• Moving between email applications: MBOX files can be used to transfer or import messages into another email application.

• Keeping an email archive: You might want to preserve old messages for future reference without keeping them in your current email account.

• Recovering information: An old archive might contain an important message, invoice or attachment you need to find.

The difficulty is that an MBOX file isn’t designed to be browsed like an ordinary document.

You can use a compatible email application to open or import the archive, but installing and configuring a full email client can feel like unnecessary work if you only want to find a message or retrieve an attachment.

That’s where a dedicated MBOX viewer can help.

What is MBOX Viewer?

MBOX Viewer runs in your browser and is designed to open email archives from services and applications such as Gmail, Thunderbird and Apple Mail.

Once you’ve opened your MBOX file, you can browse messages, search by sender or subject, filter attachments and export emails into different formats.

The main appeal is that you don’t need to set up a separate email client just to look through an email archive.

The developer also says that MBOX file contents are processed locally in your browser rather than uploaded to its servers. I wanted to take a closer look at that claim, particularly from a privacy perspective.

Testing the free version

I started with a small test MBOX file containing dummy emails. MBOX Viewer opened it successfully and displayed the messages.

Searching by sender and subject worked as expected. However, I couldn’t get it to search the body text of messages. I tried this with Pro as well, with the same result.

This means you can find an email if you know who sent it or have an idea of the subject, but searching for a phrase buried in the message itself wasn’t possible in my tests.

The free version also let me export individual messages as CSV, PDF, PST, EML and MBOX files.

I checked the CSV and PDF exports, and both contained the expected information. I also opened an exported EML message that contained an attachment and confirmed that its attachment was retained.

The developer advertises support for MBOX files up to 50 GB in the free version. I haven’t tested that limit or performance with a large archive, so I can’t comment on how well it handles files of that size.

For basic tasks such as opening an archive, looking through emails and exporting individual messages, the free version offers quite a bit without requiring payment.

Working with attachments

Attachments are often the reason for digging through an old email archive. You might be looking for an invoice, a document someone sent you or a PDF you need to keep.

MBOX Viewer lets you filter attachments by type, including images, PDFs, documents and spreadsheets. It also estimates how many attachments match your selection and their combined size before you export them in a ZIP file.

The filtering and size estimates are available in the free version, so you can see how many attachments match your criteria and how much space they take up. To download individual attachments or export bulk attachments as a ZIP file, you’ll need Pro.

I found this useful. If you have years of email containing documents you want to retrieve, being able to gather attachments in one go is much easier than opening messages individually.

What does Pro add?

At the time of testing, MBOX Viewer advertised Pro for a one-time payment of $29 (approximately £22), with no recurring subscription.

The free version supports MBOX files up to 50 GB, while Pro removes the file-size limit and unlocks bulk email exports, direct attachment downloads and bulk attachment extraction.

Is it private?

This was the part I paid particular attention to.

The developer says MBOX Viewer processes mailbox contents locally in the browser and doesn’t upload the MBOX file to its servers. Its privacy policy explains the developer’s approach to data handling.

I opened Chrome Developer Tools and monitored the Network tab while opening and searching my dummy mailbox. During those actions, I didn’t see any additional network requests triggered by the mailbox operations. I did observe requests associated with Google Analytics and other website services while using the site.

These observations are consistent with local mailbox processing, but they aren’t a comprehensive security audit. They don’t prove that no data could ever be transmitted under different circumstances, and I didn’t test every possible operation or examine every aspect of the application.

If you’re planning to open a sensitive email archive, read the privacy policy and consider your own security requirements before proceeding. My testing was encouraging, but it shouldn’t be taken as proof that the application is suitable for every type of sensitive data.

What are the limitations?

There are a few things worth knowing before you use MBOX Viewer:

• It is intended for desktop browsers, with Chrome and Edge listed as supported options.

• Message body text search didn’t work in my testing, including with Pro.

• Bulk exports and bulk attachment extraction require the paid version.

• I haven’t tested performance with very large archives or verified the advertised file-size limits.

None of these makes the tool unsuitable, but they help define what it’s best at: opening an MBOX file, finding emails using sender or subject details, and exporting messages and attachments without setting up a separate email client.

Would I use MBOX Viewer?

Yes. MBOX Viewer will definitely be going into my toolkit.

It opened my test MBOX file without any fuss, the individual exports worked, and the Pro attachment features make it a useful option when I need to retrieve files in bulk from an email archive.

Body text search would be a welcome addition, but it doesn’t take away from what the tool already does well. For the sort of archive browsing and exporting I tested, it proved straightforward to use.

If you’ve got an MBOX file sitting around that you can’t easily open, MBOX Viewer is worth a look.