MBOX Viewer

Why an mbox file is so hard to open

Most people meet the mbox format exactly once. They leave a job, close a Google Workspace account, or finally decide to move off Gmail, and Google Takeout hands them a file called All mail Including Spam and Trash.mbox. It is often several gigabytes. Double-click it and Windows asks which app should open it. macOS might offer Apple Mail, which then imports the whole thing into your live mailbox. That's rarely what you wanted.

The format itself is ancient and simple. It's every email you ever had, glued end to end in one plain text file. Each message starts with a line beginning with From followed by the raw headers, then the body, then the next message. Attachments are in there too, encoded as long blocks of base64 text. A mailbox with twenty thousand emails and a few years of PDFs and photos easily passes 5 GB. Notepad gives up. VS Code gives up. Most online viewers try to read the entire file into memory at once and the tab crashes somewhere around the first gigabyte.

The usual advice on forums is to install Thunderbird, add the ImportExportTools NG extension, create a local folder and import the file. It works, but it takes twenty minutes if you've never done it, and you end up with a mail client you'll uninstall the same afternoon. If you only need to find one contract, one receipt or one email from a person you've lost touch with, that's a lot of ceremony.

What this viewer does with it

This viewer reads the file in 8 MB pieces, the way a video player streams a film. It never holds the whole mailbox in memory. As it goes, it notes where each email starts and pulls out the sender, the recipients, the subject, the date and, for Gmail exports, the labels. A progress bar shows how far it has got. When it reaches the end you get a list of every message, newest first, with a search box and a label filter on top.

Click an email and only that message is read back from the file. The viewer decodes it properly: multipart bodies, base64 and quoted-printable encoding, the odd =?UTF-8?B?...?= subject lines that look like gibberish when you open the raw file, and non-Latin character sets such as Shift_JIS or GB2312. Attachments show up as buttons with their real file names and sizes. Click one and it downloads straight from your own file.

In my own test, a 1 GB mailbox holding almost 600,000 short emails was indexed in about 22 seconds in Chrome on an ordinary laptop, and after that every search came back instantly. Real Takeout files have fewer, bigger emails, so they index at least as fast per gigabyte. The slow part is usually your disk. An SSD is much quicker than a USB stick.

Getting your mail out of Gmail with Takeout

If you ticked Location History in the same Takeout request, the Google Maps Timeline viewer opens that part of the archive the same way, without uploading it.

Go to takeout.google.com, click Deselect all, then scroll down and tick Mail. You can click All Mail data included if you only want certain labels. Choose a zip export, wait for the email from Google (big mailboxes can take a day), download the archive and unzip it. Inside you'll find Takeout/Mail/ and the mbox file. Drop that file into the box above. Don't drop the zip itself: a multi-gigabyte zip can't be streamed the same way, so unzip it first.

Gmail exports carry one extra header that most viewers ignore: X-Gmail-Labels. It lists every label the message had, including Inbox, Sent, Starred, Important and your own labels. This viewer reads it and builds the label dropdown from it, with a count next to each one. Want only what you sent in 2019? Pick Sent, sort oldest first, and scroll. It's a small thing, but it turns one huge undifferentiated file back into something that feels like your inbox.

Thunderbird and Apple Mail files work too. Thunderbird keeps each local folder as an mbox file with no extension, named Inbox or Sent, in your profile folder. Apple Mail's Export Mailbox option creates a folder ending in .mbox with a file called mbox inside. Choose that inner file. A single saved .eml message also opens fine.

Reading, searching and saving messages

Already have a folder of saved .eml files and want them as PDFs? The EML to PDF converter handles those without opening each one.

The search box matches sender names, addresses, recipients and subjects, and it filters the list as you type. It doesn't search inside message bodies. Doing that across 5 GB would mean rereading the whole file for every keystroke, and I'd rather give you an instant header search than a slow full-text one. In practice, the sender plus a word from the subject finds the email you're after almost every time.

HTML emails are shown in a locked-down frame. Scripts can't run, forms are stripped, and links open in a new tab. Remote images are blocked by default. That matters more than it sounds: newsletters and marketing mail are full of tracking pixels, and loading them tells the sender exactly when the email was opened, even years later. If an email looks broken without its pictures, click Load them and they'll appear. Images attached inside the message itself are always shown, because they come from your file and not from the internet.

Each message has two save options. Save as EML downloads that one email as a standard .eml file with all its attachments intact, which Outlook, Apple Mail and Thunderbird all open. Print or PDF opens your browser's print dialog with a clean copy of the message and a header block showing who sent it, to whom and when. Choose Save as PDF as the printer and you've got a PDF you can hand to an accountant, a lawyer or HR.

Privacy: why nothing gets uploaded

A mail archive is about the most private file most people own. Bank statements, medical letters, password reset links, years of personal conversations. Uploading it to a website you found five minutes ago would be a strange decision, and you shouldn't have to. Everything here happens in JavaScript inside your browser tab. The file is read from your disk with the browser's own file API and nothing is sent to this site or anyone else. If you want to check, load the page, turn off your Wi-Fi, and open your mailbox. It still works.

When you close the tab, the index is gone. There's no account, no history and no copy kept anywhere.

Where it falls short

It's a viewer, not a mail client. You can't reply, move messages between folders, or edit anything, and the original file is never changed. It reads mbox and eml only. Outlook PST files are a different, binary format and won't open here. Very old mailboxes written by unusual software sometimes skip the escaping that mbox needs, and an email that quotes a line starting with the word From can then get split in two. It's rare, and the rest of the mailbox reads normally when it happens. For everything else, from finding a single old receipt to skimming ten years of mail before deleting an account, it should save you an afternoon.