If you have ever backed up your email, or downloaded your data from Google, you have probably ended up with a file ending in .mbox and no idea what to do with it. This guide explains, in plain language, exactly what that file is, why it exists, and how to open it, without drowning you in technical jargon.
What exactly is an mbox file?
At its heart, an mbox file is just an ordinary text file. What makes it special is that it holds not one but many emails, stored one after another. Picture a long paper scroll onto which you glue hundreds of letters in sequence: that is the idea behind mbox. Instead of a thousand separate files for a thousand emails, you have a single file that keeps everything together.
The name mbox is short for "mailbox". The format has been around since the 1970s, which is why almost every email program recognises it. That long history is also its biggest strength: it is an open, simple format that is not locked to any single brand.
How does mbox fit all those emails into one file?
The trick is surprisingly simple. Every new email in the file begins with a special line that starts with the word From (followed by a space). That line acts as a separator: the moment an email program sees it, it knows a new message starts there.
Inside each message there is first a block of technical information (the so-called headers, such as sender, recipient, subject and date) and below that the actual text of the email. Attachments such as photos and PDFs are converted into text characters and written into that same file. This is why an mbox file with lots of attachments can grow quite large.
Worth knowing
Because everything lives in a plain text file, an mbox file stays readable and usable even ten or twenty years from now. You are not tied to one particular program that might no longer exist by then. That makes mbox an excellent choice for long-term archives.
Which programs use mbox?
You will run into the mbox format in surprisingly many places. The best known are:
- Mozilla Thunderbird: one of the most popular free email programs. It stores all your folders as mbox files.
- Apple Mail (macOS Mail): when you export a mailbox on a Mac, you get back a
.mboxfolder. - Google Takeout: when you request your Gmail data, you receive a large mbox file, often packed as
.mbox.gz. - Older Unix and Linux mail servers: these used mbox as a default for decades.
Because so many programs support mbox, it is a handy format for moving email from one program to another.
Mbox versus eml versus pst: what is the difference?
Alongside mbox, you will often see two other names. It helps to know the difference.
| Format | What it holds | Typically used by |
|---|---|---|
| mbox | Many emails together in one text file | Thunderbird, macOS Mail, Google Takeout |
| eml | A single email per file | Windows Mail, Outlook (save as), general use |
| pst | Many emails plus calendar and contacts, in a closed Microsoft format | Microsoft Outlook |
In short: an .eml file is one standalone email, whereas mbox deliberately packs many into a single file. Outlook's .pst file is similar in purpose to mbox, but it is a closed Microsoft format that can also hold calendar appointments and contacts. By comparison, mbox is far more open and straightforward.
How do you open an mbox file?
Double-clicking an mbox file usually does not work: Windows or macOS does not know what to do with it, or opens it as an unreadable wall of text. So you need a program that understands the format. A handy option is the free Chrome extension Mbox Viewer by Cloud Captains, which you can find at mbox-viewer.online. What makes it special is that everything happens on your own device.
Privacy first
Mbox Viewer by Cloud Captains works entirely locally and offline. Your emails are never uploaded to a server, there is no cloud, and nothing is tracked about what you read. All data stays in your own browser's storage, and you can wipe it yourself at any time via Settings, Clear database.
Opening an mbox file in Mbox Viewer
- Install the Mbox Viewer by Cloud Captains extension from the homepage mbox-viewer.online.
- Open the viewer in a new tab.
- Drag your
.mboxfile from your folder onto the window (drag and drop). - Wait for the emails to appear in the list on the left.
- Click a message to read it on the right, just like you are used to in Gmail or Thunderbird.
Besides mbox, the viewer also opens variants and related formats: .mbx, .eml, .emlx (Apple Mail), .msg (Outlook), Maildir folders and .mbox.gz from Google Takeout. That last, compressed file is unpacked automatically, so there is nothing you need to do yourself.
Viewing and searching an mbox file
Once your file is open, you see the list of emails on the left and the selected message on the right. Each message has three tabs: Preview (the email as you would normally read it), Raw (the unprocessed text including all technical headers) and Forensic (a deeper analysis for anyone who wants to check whether an email is genuine).
Searching is fast thanks to handy operators. A few examples:
from:johnsearches by sender.subject:invoicesearches by subject.has:attachmentshows only messages with an attachment.after:2024-01-01shows only recent emails."exact phrase"searches for a precise combination of words.
You can also organise emails with labels, stars and short notes, so you can quickly find what mattered later on.
External images stay blocked at first
While reading, external images are not loaded by default. This is deliberate: some images are in fact invisible tracking pixels that let a sender see that you opened an email. If you do want to see the pictures, you load them yourself per message with a button.
Frequently asked questions
Is an mbox file dangerous to open?
No, the file itself is just text and cannot run anything on its own. It may, however, contain emails with suspicious links or attachments, just like your regular inbox. So open unknown attachments with the same caution you always would.
Can I open an mbox file without uploading my emails?
Yes. With a local viewer such as Mbox Viewer by Cloud Captains, everything stays on your own device. Nothing goes to a server or the cloud, so your privacy stays entirely with you.
How large can an mbox file get?
That depends on the number of emails and their attachments. A backup of a busy mailbox can quickly reach several gigabytes, especially when it contains many photos and PDFs.
What is the difference between mbox and mbx?
They are practically the same. .mbx is simply another file extension that some programs use for the same mailbox format. Mbox Viewer opens both without any trouble.
Can I pull individual emails out of an mbox file?
Yes. In Mbox Viewer you can select one or more messages and export them, for example as a standalone .eml file, as a PDF, or as a new, smaller mbox file containing only the messages you need.