What is Mbox Viewer?
Mbox Viewer by Cloud Captains is a free Chrome extension that opens and searches old email archives without importing them into a mail program. Think of a Google Takeout or Google Vault export, a Thunderbird or Apple Mail backup, a complete Outlook mailbox, or a single message someone sent you. You drag the file into the window and read your messages straight away, just like looking at Gmail or Thunderbird.
The extension runs on Manifest V3, the modern Chrome standard with minimal permissions. The mascot is Captain Frank, a friendly pirate who guides you through your archive. The latest release is version 2.2.3 (August 2026).
100 percent local and free
Your files never leave your device. All processing happens in your browser: no uploads, no trackers, no ads, no hidden costs. Every feature is free.
Installing
Install Mbox Viewer
- Go to the install page at cloud-captains.com/mbox-viewer.
- Continue to the Chrome Web Store and choose Add to Chrome.
- Confirm the install. The extension appears in your toolbar.
- Click the icon to open Mbox Viewer in its own tab.
Opening your first archive
There are two ways to open an archive: drag a file or a whole folder into the window, or use the browse button. A whole folder of individual messages or a Maildir structure with the cur and new subfolders opens in one go.
Open an email archive
- Open Mbox Viewer from the toolbar icon.
- Drag your file or folder onto the large area in the middle.
- Wait for the progress window to finish. For large archives you see a live log with speed, encoding and timing.
- Browse the message list on the left and click a message to read it on the right.
Which files can you open?
| Format | Files | Origin |
|---|---|---|
| Single email | .eml, .emlx | Outlook, Apple Mail, saved messages |
| Mbox | .mbox, .mbx, .mbox.gz | Google Takeout, Google Vault, Thunderbird, Unix, gzip |
| Outlook | .pst, .ost, .msg | Complete Outlook mailbox, including folder structure and winmail.dat |
| Maildir | folders with cur and new | Dovecot, Courier, Linux mail servers |
| Archive | .zip, .tar.gz | Any of the above, packed |
You open Outlook pst and ost files directly, with no export or conversion first. The original folder structure, winmail.dat attachments and embedded Outlook messages are preserved. From pst and ost the viewer also brings in contacts, calendar appointments, tasks and notes, with a filter by item type.
The three tabs on a message
On the right, every open message has three tabs:
- Previewshows the message cleanly formatted, with inline attachments. External images and hidden CSS trackers are blocked by default.
- Forensicshows SPF, DKIM and DMARC verdicts, a suspicion score, hashes and the route the message travelled.
- Rawshows the full, unprocessed source of the message with every header.
Large archives
There is no real size limit. For a large file, Mbox Viewer automatically switches to its built-in local database (IndexedDB) on your own device. You never have to split anything and every message stays local. Search results appear instantly while large mailboxes keep scanning in the background.
Do I have to create an account?
No. Mbox Viewer works entirely without an account and without an internet connection. You install the extension and start straight away.
Do my emails ever leave my device?
No. All processing happens in your browser. No files are uploaded and there is no tracking or telemetry.
Can I open a whole folder of individual messages?
Yes. Drag the folder into the window. Individual eml files and Maildir folders with cur and new are read in one go.
Does the extension work in other browsers?
Mbox Viewer is built for Chrome and Chromium-based browsers such as Edge. Installation runs through the Chrome Web Store.
Want to learn more? Continue with searching and filtering or read how to export emails and build a forensic case file.