What is Mbox Viewer?
Mbox Viewer by Cloud Captains is a free Chrome extension that lets you open and search old email archives without importing them into a mail program first. Think of a Thunderbird export, an Apple Mail backup, an Outlook message someone sent you, or a Google Takeout download of your Gmail. You drag the file into the window and start reading straight away, just like looking at Gmail or Thunderbird.
The mascot is Captain Frank, a friendly pirate who guides you through your archive. The extension is built on Manifest V3 (the modern Chrome standard) and is at version 1.6.5 at the time of writing.
Why is this useful?
Email archives are often awkward to open. You do not want to import a multi-gigabyte .mbox file into your everyday mail client, and a single Outlook .msg file does not always open neatly on a Mac. Mbox Viewer solves that: you open the file on its own, read what you need, and close it again. Nothing about your real mailbox changes.
The biggest difference from online tools is privacy. Mbox Viewer runs 100% locally.
100% local and offline
Your emails never leave your device. There are no uploads, no servers and no telemetry. The extension's Chrome permissions are empty. All data stays in IndexedDB on your own computer, the local database vault that the browser manages. You can wipe everything at once through Settings, Clear database.
The only time any network traffic can occur is if you actively choose to embed external images in a PDF export. While simply reading, that never happens.
You can find more information and downloads on the homepage mbox-viewer.online, built by Cloud Captains.
Which files can I open?
Mbox Viewer recognises most of the email formats you run into in the wild:
.mboxand.mbx, the export format of Thunderbird and macOS Mail.eml, the standard single-message format.emlx, the message format used by Apple Mail.msg, the message format used by Microsoft Outlook- Maildir folders, where each message is a separate file
.mbox.gz, the compressed format from Google Takeout, which is unpacked automatically
Is your Gmail export sitting inside a compressed .mbox.gz file? Then you do not have to unpack anything by hand, the extension does it for you.
Installing and opening
Install Mbox Viewer and open your first archive
- Go to the homepage mbox-viewer.online and follow the link to the Chrome Web Store.
- Click Add to Chrome and confirm the installation. Because the permissions are empty, Chrome will not ask for access to your data.
- Open the extension using the Mbox Viewer icon in your browser. The main window appears.
- Drag an email file or a Maildir folder from your file explorer or Finder into the window and drop it.
- Wait for the messages to load. With a Google Takeout file, the
.mbox.gzis unpacked automatically first. - Your archive now appears in the message list on the left. Click a message to read it on the right.
Several files at once
You can drag several files into the window at the same time. Handy when your archive is spread across multiple export files.
The layout: list on the left, message on the right
The layout looks like Gmail or Thunderbird, so you feel at home right away.
- On the left sits the message list with sender, subject and date. This is where you browse and select.
- On the right sits the open message. At the top you see three tabs that let you view the same message in different ways.
The three tabs per message are:
- Preview: the message as it was meant to look, with formatting and images, neatly readable.
- Raw: the full original source text of the email, including every header. Useful when you want to see exactly what was sent.
- Forensic: a deeper analysis of the message, including the authenticity check and a suspicion score. This tab is there for anyone who wants to verify whether a message can be trusted.
External images are blocked by default
While reading, external images are not loaded automatically. This protects you from tracking pixels, tiny invisible images that let a sender see that you opened an email. If you do want to see the images, you load them per message with a button. You stay in control.
Reading, sorting and browsing messages
Click a message in the list to open it. For anyone who wants to move quickly through a large archive, there are keyboard shortcuts that work the same way as in many mail programs.
| Key | What it does |
|---|---|
j / k |
Move to the next or previous message |
Enter |
Open the selected message |
x |
Select a message (for example for a bulk export) |
Shift + A |
Select all messages |
/ |
Jump to the search field |
? |
Show the help with all shortcuts |
Esc |
Close the current message or window |
Organise as you read
When you come across an important message, you can star it right away, attach a label or jot down a Captain note. That way you find it again later without combing through your archive a second time.
If you want to search precisely, press / and type a search term. Mbox Viewer also understands handy operators such as from:, subject:, has:attachment and after:2024-01-01, but we cover those in detail in a separate article.
Settings worth a quick look
Through Settings you tune the extension to your preference. A few things you probably want to set right away:
- Language: the interface is available in twelve languages.
- Theme: light, dark or following your system.
- Accessibility: more contrast and reduced motion for anyone who benefits from it.
- In-memory limit: this controls how much of a large archive may use your working memory.
You can also export your entire workspace and import it again later, for example to bring your labels and notes to another computer.
Ready to start
Install the extension, drag your first file into the window and browse your messages with j and k. Everything stays on your own device, so you can safely open a sensitive archive without anything going to the cloud.
Are my emails uploaded anywhere?
No. Mbox Viewer works entirely locally and offline. There are no uploads, no servers and no telemetry, and all data stays in your browser's local database on your own device. The only possible network action is optional: embedding external images when you export a PDF yourself.
Which file types can I open?
You can open .mbox and .mbx, .eml, .emlx, .msg and Maildir folders. A Google Takeout file in .mbox.gz works too, and it is unpacked for you automatically.
How do I load a file?
You drag the file or Maildir folder from your file explorer or Finder into the extension window and drop it. The messages then appear in the list on the left.
Why do not I see images in an email?
External images are blocked by default while reading to stop tracking pixels. A button inside the message lets you load them anyway whenever you want.
How do I wipe everything again?
Go to Settings and choose Clear database. That removes all loaded messages, labels and notes from your device at once.
Where can I find more information?
Visit the homepage mbox-viewer.online or the maker Cloud Captains at cloud-captains.com.