Seeing an error code while loading a mailbox? Mbox Viewer runs entirely in your browser, so you can fix most issues yourself without any data leaving your device. Find your code below and follow the steps.
How to find an error code
Every error shows a code such as ERR-PARSE-001. The same codes are listed under the Support tab in the extension. While loading, the progress window also shows a live log with bytes, mails per second, encoding and timing. Open DevTools with F12 to see the full log and attach it when you report a bug.
ERR-PARSE-001: the file cannot be read
Cause. The file could not be parsed. It is most likely corrupt, truncated, or not a valid MBOX or EML format.
Fix ERR-PARSE-001
- Check that the file downloaded completely. An interrupted download from Google Takeout or a mail client is the most common cause.
- Re-export the mailbox from the source, for example through Takeout for Gmail or File export in Thunderbird.
- Check the format. Supported types are
.mbox,.mbx,.eml,.emlx, Maildir folders and compressed.mbox.gz. - Open a small test file to confirm the viewer itself works. If it does, the original file is the problem.
ERR-DB-001: the local database cannot be opened
Cause. Mbox Viewer stores your mails locally in IndexedDB. This error means storage is full or IndexedDB is blocked by the browser or a policy.
Fix ERR-DB-001
- Check your browser storage, in Chrome through Settings, Privacy and security, Site settings.
- Close other tabs that have the extension open. Several tabs at once can lock the database.
- Use Clear database in the extension and load the mailbox again.
- Allow persistent storage when the browser asks, otherwise the browser may evict your data under storage pressure.
- Do not test in an incognito window, where IndexedDB is often limited or temporary.
ERR-MEM-001: the file is too large for memory
Cause. The file does not fit in the available memory.
Fix ERR-MEM-001
- Let the viewer switch to database mode automatically. For large files this happens on its own.
- Close heavy tabs and programs to free up memory.
- If it still fails, split the mailbox into smaller parts.
ERR-FILE-TOO-LARGE: file larger than 2 GB
Cause. The file exceeds the 2 GB limit.
Fix ERR-FILE-TOO-LARGE
- Split the MBOX file into smaller parts with an mbox splitter.
- Load the parts one by one. Tags and notes are kept per mail.
ERR-UNSUPPORTED: file format not supported
Cause. The format is not supported yet, for example .pst or .ost from Outlook.
Fix ERR-UNSUPPORTED
- Export to MBOX in Outlook first, or use a PST to MBOX conversion tool.
- Load the resulting MBOX file. Support for PST and OST is on the roadmap for v1.7.
ERR-CRYPTO: encryption not available
Cause. The browser has no Web Crypto or AES-256, which are needed for the encrypted forensic case file.
Fix ERR-CRYPTO
- Use an up-to-date version of Chrome or Edge.
- Make sure you open the viewer over a secure HTTPS context. Web Crypto does not work on an insecure connection.
Known browser messages that are not errors
Blocked script execution in 'about:srcdoc'
With DevTools open, Chrome logs this message every time an email is rendered. The preview runs inside a sandboxed iframe without allow-scripts, precisely to protect you from any JavaScript in mail content. Chrome prints the message for every sandboxed iframe, even for mail with no script at all. The impact is zero and users without DevTools never see it.
Frequently asked questions
Does my data ever leave my device when an error occurs?
No. All processing happens locally in your browser. Even on an error, nothing is uploaded.
Where do I find the error codes in the extension?
The Support tab in Mbox Viewer lists the same codes and fixes.
How do I report a bug that is not listed here?
Go to mbox-viewer.online for bug reports and the full FAQ, or email support. Include a short description, the steps to reproduce, and the DevTools log if you can.
Which file formats can Mbox Viewer read?
.mbox, .mbx, .eml, .emlx, Maildir folders and compressed .mbox.gz. PST and OST will follow later.