Frequently asked questions
Everything you might want to know before opening your .mbox file in the browser.
- Is the online MBOX viewer safe to use?
- Yes. Your file is read entirely inside your browser — it is never uploaded to a server. Message HTML is sanitized with DOMPurify and rendered in a sandboxed iframe, and remote images are blocked by default to stop tracking pixels.
- Is my email uploaded anywhere?
- No. There is no upload and no backend. The whole viewer runs as JavaScript in your browser, so the contents of your .mbox file never leave your device.
- What is the maximum file size?
- The online viewer handles files up to 25 MB so it stays fast in the browser. For larger archives — full Gmail Takeout exports, for example — use the desktop app, Mbox Viewer for Mac, which streams files of any size.
- Which email apps export .mbox files?
- Apple Mail (Mailbox ▸ Export Mailbox), Mozilla Thunderbird (with the ImportExportTools NG add-on) and Google Takeout (which exports your Gmail as a single .mbox) all produce .mbox files. Many other clients can import or export the format too.
- Can it open .eml files too?
- Yes. A single .eml message is opened as a one-message archive. The viewer also reads Gmail labels stored in the X-Gmail-Labels header and lets you filter the message list by label.
- Does it work offline?
- Once the page has loaded, parsing happens locally, so reading your file does not need a connection. The first page load does require the internet to fetch the site.
- Why are some images not shown?
- Remote images are blocked by default for your privacy, because marketing emails often use them as tracking pixels. A bar at the top of a message lets you load the images for that message if you trust the sender.
- Is it really free?
- Yes, the online viewer is completely free and open source (MIT licensed). The optional desktop app for Mac is a separate product for power users who need to open very large archives.