# Mbox vs eml vs maildir: which email format is best? ## Mbox, eml and maildir in a nutshell When you export email from a mail program or make a backup, you will almost always run into one of these three formats: mbox, eml or maildir. All three do the same basic job (storing email on your own disk), but the way they do it differs greatly. That difference decides how robust your archive is, how high the risk of damage is, and how pleasant it is to work with in practice. In this article we explain in plain language what each format is, where it is strong and weak, and when to pick which one. Then we show how to open all three locally with Mbox Viewer by Cloud Captains, without a single byte ever leaving your computer. :::info title="Everything stays on your own device" Mbox Viewer is a Chrome extension that works 100% locally and offline. Nothing goes to a server or the cloud. Whichever of these three formats you use, your email stays on your own device, stored in the browser database (IndexedDB). More info: https://mbox-viewer.online ::: ## What is mbox? Everything in one file With the mbox format, all of your messages sit one after another inside a single text file. Picture a long diary in which each message is neatly pasted below the previous one, with a separator line in between. That whole diary is an mbox file. Mail programs such as Thunderbird and macOS Mail use it, and Google Takeout also hands you your Gmail archive as mbox. The strength of mbox is convenience and compactness. An entire mailbox, sometimes tens of thousands of messages, fits in one tidy file that you can easily copy, move or place on an external drive. For archiving, that is ideal. Its weakness is the flip side of that strength: everything is in one file. If that file gets damaged in an unlucky spot (through an interrupted copy, a full disk or a write error), the damage can affect several messages instead of just one. Because messages are separated by a text line starting with "From ", a message that itself contains such a line can, in rare cases, cause confusion about where one message stops and the next begins. :::tip title="Google Takeout? No need to unzip" Google often hands you your Gmail archive as .mbox.gz, a packed (compressed) version. Mbox Viewer automatically unpacks it for you as soon as you drag the file into the window. So you do not need to unzip it yourself first. ::: ## What is eml? One file per message With eml, every email message gets its own file. A folder full of .eml files is therefore literally a stack of loose emails, one per file. Apple Mail uses a similar variant (.emlx) and Outlook its own .msg format, but the idea is the same: one message, one file. The big win with eml is safety and clarity. Because every message stands alone, a damaged file only affects that single message. The rest stays perfectly readable. If you want to forward a single email, keep it as evidence or hand it to someone, you simply grab that one file. That makes eml popular for legal matters and individual exports. The downside: with large volumes you end up with a lot of separate files. A mailbox of 20,000 messages becomes 20,000 files, which is less practical to move or back up in one go than a single mbox file. ## What is maildir? A folder structure Maildir resembles eml (also one file per message), but adds a clever folder structure designed precisely for robustness. A maildir consists of three subfolders: "new" for freshly arrived mail, "cur" for messages you have already viewed, and "tmp" for messages still being written. Every message gets a unique file name. That setup makes maildir the most robust of the three against corruption. A message is first written safely into "tmp" and only then moved, so a half-written message never pollutes your archive. Because each message is its own file, a write error, just like with eml, can never hit several messages at once. That is why mail servers favour maildir, since multiple processes can safely write into it at the same time. The downside is the same as eml plus a touch more: it is many files, spread across several folders, which makes it a little less tidy to browse by hand. ## The three formats side by side | Feature | Mbox | EML | Maildir | | --- | --- | --- | --- | | Storage | Everything in one file | One file per message | One file per message in folders | | Robustness | Reasonable | Good | Best (designed against corruption) | | Risk of corruption | Damage can hit several messages | Damage stays with one message | Damage stays with one message | | Ease of archiving | Best (one file) | Less (many files) | Less (many files plus folders) | | Sharing a single email | Awkward (you must extract it) | Best (grab one file) | Good (grab one file) | | Typical use | Archive, Gmail export, Thunderbird | Individual exports, evidence | Mail servers, robust storage | ## Which format do you choose when? There is no format that always wins. The best choice depends on what you want to do. - Want to archive a whole mailbox or keep a Gmail export? Choose mbox. A compact file is pleasant to store away and find again. - Want to keep, forward or submit a few specific emails as evidence? Choose eml. Each file stands on its own and is easy to share. - Want maximum protection against damage, for example a long-term archive you really cannot afford to lose? Choose maildir, the format designed for exactly this. :::warn title="Always keep a second copy" Whichever format you choose: no format protects you against a broken disk or an accidentally deleted folder. The golden rule remains a backup in a second place. Thanks to its single-file setup, mbox is the easiest to quickly tuck away somewhere safe. ::: ## How do you open all three locally? The nice part is that you do not need to understand the technical differences to read them. Mbox Viewer opens all three formats in the same familiar screen: the message list on the left, the opened message on the right, just like Gmail or Thunderbird. Besides mbox, eml and maildir, the extension also recognises .mbx, .emlx (Apple Mail), .msg (Outlook) and .mbox.gz (Google Takeout). :::howto title="Opening email in Mbox Viewer" 1. Install Mbox Viewer by Cloud Captains from the Chrome Web Store and open the extension. 2. Grab your file or folder: for mbox drag the .mbox file, for eml one or more .eml files, and for maildir drag the entire maildir folder (with the subfolders new, cur and tmp). 3. Drag your selection into the window (drag and drop). A .mbox.gz file is unpacked automatically. 4. Wait for the messages to appear in the list on the left. Everything is read in locally on your own device, nothing goes to the internet. 5. Click a message to read it on the right. Browse quickly with the j and k keys, and open with Enter. ::: :::tip title="Shortcuts make it fast" Press the question mark (?) for the full overview. The handiest ones: j and k to browse, Enter to open, x to select, the slash (/) to search and Esc to close. ::: ## Privacy: why opening locally matters Email often holds your most sensitive information. Many online viewers and conversion services make you upload a file to their server, and then you no longer have any idea who can read your messages. Mbox Viewer deliberately goes the other way. - The extension's Chrome permissions are empty, there is no default network access. - Your email is stored in the browser database (IndexedDB) on your own device, not in a cloud. - There is no telemetry and no data is sent anywhere. - External images are blocked by default while reading, so hidden tracking pixels cannot follow you. You only load them when you click for them yourself. - Want to wipe everything? Go to Settings and choose Clear database. The only time optional network access comes into play is when you yourself want to embed external images in a PDF export. If you do not do that, everything stays fully offline. :::faq ### Which email format is most robust against damage? Maildir is the most robust, because it is designed against corruption: messages are written safely first and only then moved, and each message sits in its own file. Eml also handles damage well, since a faulty file only affects that single message. With mbox, damage can hit several messages at once because everything is in one file. ### Which format is handiest for keeping a whole archive? Mbox, because a complete mailbox fits in a single file. That is far easier to copy, move and back up than thousands of separate files. For sharing individual emails, eml is the more convenient choice. ### Do I have to unzip a .mbox.gz from Google Takeout myself first? No. Mbox Viewer unpacks a .mbox.gz automatically as soon as you drag it into the window. You simply drag the file Google gives you and the messages appear on their own. ### Can I switch between formats, for example mbox to eml? Yes. Open your files in Mbox Viewer and export them in a different format. Per message you can export to PDF, .eml, .html or .mbox, and in bulk to options such as an EML-ZIP or HTML-ZIP. That way you can, for instance, turn an mbox archive into individual eml files. ### Does my email leave my computer when I open these formats? No. Mbox Viewer works 100% locally and offline. Your files are read into the browser database on your own device and there is no telemetry. Only if you choose to embed external images in a PDF might optional network access be needed. ### Which format should I choose for evidence or legal matters? Eml is popular for this, because each message is a separate, self-contained file that you can easily keep and share on its own. If you want to capture several messages together with hashes and an overview, Mbox Viewer also offers a Forensic case file, a ZIP with the emails, forensic PDFs, hashes and a manifest, optionally protected by an AES-256 password. :::