What Mbox Viewer is and why privacy matters here
Mbox Viewer by Cloud Captains is a Chrome extension (Manifest V3, version 1.6.5) that lets you open and investigate email archives without your email ever leaving your computer. Captain Frank, the app's pirate mascot, guards your treasure: your messages stay yours.
The app is built for people who handle sensitive email: journalists, lawyers, IT administrators, researchers and anyone digging through an old mailbox. For that group it is essential that nothing ends up in an unknown cloud. That is exactly why Mbox Viewer is 100% local and offline.
You can read more about the app on the homepage: https://mbox-viewer.online
What working locally looks like in practice
Here "local" does not mean "mostly local". It is complete:
- No uploads. Your files are never sent anywhere, to anyone.
- No servers. The app needs no backend server to do its job.
- No telemetry. Nothing tracks what you do or open.
- Empty Chrome permissions. By default the extension requests no rights at all over your browser or system.
- Everything in IndexedDB. That is storage inside your own browser, on your own device. Nothing lives anywhere else.
There is exactly one exception, and you turn it on yourself: if you want to embed external images in a PDF export, the app can fetch those images from the internet. Outside that single optional case there is no network access at all.
Empty permissions, what that means
Many extensions ask for rights like "read your browsing history" or "change data on all websites". Mbox Viewer asks for none of that. Its permission list is empty, so the extension simply cannot peek at your other tabs.
External images: blocked by default
Many emails contain images that are not part of the message itself but are loaded from the sender's server. That sounds harmless, yet such images are often used as a tracking pixel: an invisible graphic that tells the sender you opened the mail, when, and sometimes where.
Mbox Viewer blocks these external images by default while you read. That way nobody can tell that you opened an old message. If you do want to see the pictures, you load them per message with a button. You stay in control.
Viewing safely
Only load external images for messages whose sender you trust. For suspicious or unknown mail it is wiser to leave them blocked, so any tracking pixel stays blind.
Every setting at a glance
The settings screen lets you tailor the app to the way you work. The main options:
- Language: choose from 12 languages.
- Theme: light, dark or system (follows your computer's setting).
- Contrast: higher contrast for better readability.
- Reduced motion: turn off animations if you are sensitive to them.
- External images: decide whether images from the internet are loaded by default.
- In-memory limit: set how many messages Mbox Viewer keeps in working memory at once. With very large archives a lower limit keeps your browser from slowing down.
- Export and import workspace: back up your labels, stars, notes and settings, or restore them.
Backing up and restoring your workspace
Because everything stays local, there is no cloud quietly keeping a copy. That is great for your privacy, but it also means you are in charge of your own backups. Luckily that takes just one button.
Export your workspace as a backup
- Open Mbox Viewer and go to Settings.
- Under Workspace, choose Export.
- Save the backup file somewhere safe, for example an external drive.
- Keep this backup separate if you plan to clear your browser data or move to another computer.
Restore your workspace on a new device
- Install Mbox Viewer on the new device.
- Go to Settings and choose Import under Workspace.
- Select the backup file you saved earlier.
- Your labels, stars, notes and settings are back exactly as they were.
No automatic backup
Because Mbox Viewer keeps nothing in the cloud, there is no automatic recovery either. If you clear your browser data or lose your device without an export, your labels, stars and notes are gone. So export your workspace regularly.
Wiping the database: starting fresh
If you want to clear everything, for instance on a shared computer or after a sensitive investigation, you can wipe all local storage in one go.
Wipe the database
- Go to Settings.
- Choose Clear database.
- Confirm your choice.
- All loaded email, labels, stars and notes in IndexedDB are removed.
This cannot be undone
Clearing the database really removes everything stored locally. If you have labels, stars or notes you want to keep, export your workspace first. After wiping, recovery without a backup is impossible.
Keyboard shortcuts for faster work
Mbox Viewer is built to fly through large mailboxes. Using the keyboard is a lot quicker than the mouse.
- j and k: move to the next or previous message.
- Enter: open the selected message.
- x: select a message (for example to prepare a bulk export).
- Shift+A: select all messages.
- / : open the search field.
- ? : show the help page with shortcuts.
- Esc: close the current message or window.
Combine selecting and exporting
Use x to tick individual messages or Shift+A to grab them all, then export them in bulk as EML-ZIP, PDF-ZIP or a forensic case file. That way you handle an entire archive in just a few steps.
Frequently asked questions
Are my emails uploaded or stored on any server?
No. Mbox Viewer processes everything on your own device. There are no uploads, no servers and no telemetry. Your files stay in IndexedDB inside your own browser.
Why don't I see the images in my email right away?
External images are blocked by default to stop tracking pixels, so senders cannot tell that you opened a message. You load them per message with a button if you do want to see them, or you enable them by default in the settings.
My browser slows down with a very large archive. What can I do?
Lower the in-memory limit in the settings. That makes Mbox Viewer keep fewer messages in working memory at once, which keeps the app smooth with big mailboxes.
How do I move my labels and notes to another computer?
Export your workspace from Settings first, copy the file to the new computer and use Import there. Your labels, stars, notes and settings will come right back.
Which file types can I open?
Mbox Viewer opens .mbox and .mbx, .eml, .emlx, .msg, Maildir folders and .mbox.gz from Google Takeout, which is unpacked automatically. You simply drag the files onto the window.
Does the extension need the internet to work?
No, not for reading and investigating your email. Only when you choose to embed external images in a PDF export does the app fetch those images from the internet. Everything else works offline.