What Is an mbox File, and Why an Open Format Matters

An mbox file is a plain-text file that holds many email messages back to back, each one exactly as it arrived. Every message starts with a line beginning with From (with a trailing space), which is how a program knows where one message ends and the next begins. It’s one of the oldest and most widely used email formats: Thunderbird uses it to store local mail, Apple Mail uses it to export mailboxes, and Google Takeout uses it to hand you your Gmail. Because it’s plain text and nobody owns it, it’s a good choice for a backup you want to be able to open many years from now.

What’s inside an mbox file

Open one in a text editor (ideally one that copes with large files) and you’ll see something like this:

From jane@example.com Mon Sep 14 09:12:44 2026
Return-Path: <jane@example.com>
From: Jane Miller <jane@example.com>
To: you@example.net
Subject: October quote
Date: Mon, 14 Sep 2026 09:12:40 -0400
Content-Type: text/plain; charset=utf-8

Hi, here's what we talked about yesterday.

From news@store.example Tue Sep 15 18:03:10 2026
Return-Path: <news@store.example>
...

That first line, starting with From and no colon, is the separator. After it come the message headers (sender, recipient, subject, date), a blank line and the body. Attachments sit inside the same text, encoded in base64, which is how they always travel by email anyway. That’s why an mbox full of photos takes up a bit more space than the photos would on their own.

That’s all there is. No database, no proprietary encryption, nothing only one program understands.

An old format with no owner

mbox comes from the Unix world and has been around for decades. Oddly enough, it never had a single official specification. The reference document, RFC 4155 from 2005, registers the application/mbox media type and openly says the format lacks an authoritative spec, with each implementation differing in small ways.

The best-known difference is what to do when a line in a message body also starts with From . Left alone, a program might mistake it for the start of a new message. The usual fix is to add a > in front when saving (>From ). The mbox variants you may see mentioned (mboxo, mboxrd and others) differ mostly in how they handle this. In practice it rarely matters: modern programs read each other’s mbox files without trouble.

Which apps use and open mbox

• Thunderbird stores each local folder as an mbox file by default, with a separate index (.msf) for speed, according to its developer documentation. To open a standalone mbox file, the ImportExportTools NG add-on is the easiest route.
• Apple Mail exports and imports mailboxes in mbox format from the Mailbox menu and File > Import Mailboxes. What it exports is an .mbox package with the message file inside.
• Google Takeout delivers your Gmail as an .mbox file, with each message’s labels stored in a header.
• Dedicated viewers such as MBox Viewer Pro open large mbox files so you can search and read them without importing them into an email client.

mbox compared with other email formats

Format	What it is	Strengths	Weaknesses
mbox	Many messages in one text file	Open, opens almost anywhere, easy to move	A damaged file affects many messages; huge files are unwieldy
EML	One message per file	Very simple, opens with a double-click	Thousands of loose files; folder structure is lost unless you rebuild it
Maildir	A folder with one file per message	Robust, common on servers	Less support in desktop apps
PST	Outlook data file	Also holds calendar and contacts	Built for Outlook; Microsoft publishes the spec, but few programs outside Outlook read it well

No format is perfect. mbox wins on what matters most for a backup you’ll keep for years: being able to open it tomorrow with whatever you have on hand.

Why an open format matters for a backup

A backup that only the app that made it can open has a hidden weak spot. If that app is discontinued, changes its format or stops running on your system, your backup turns into a file you can’t read. And you tend to find out on the day you need it.

With mbox, that risk mostly goes away:

• You’re not tied to anyone. Plenty of programs read it, free and paid, on Windows, Mac and Linux.
• You can check your backup. Open the file and look at the emails. No need to trust a green checkmark.
• Ordinary tools can search it. It’s text, so even a find in a text editor will turn up a subject line or an address.
• It moves without conversion. Taking mail from Apple Mail to Thunderbird, or from a backup to another app, doesn’t require converting anything.

mbox’s weak spot, and how to avoid it

The real drawback of mbox is size. Put ten years of inbox into a single 20 GB file and every change means rewriting a huge file, and if it gets corrupted, the damage reaches a lot of messages. It’s also awkward to upload to the cloud or keep versions of.

The fix is simple: don’t put everything in one file. Split mail by folder and by period, and each file stays small, older files never change, and a problem in one doesn’t touch the rest.

How Arca uses it

Arca Mail Backup saves your mail as standard mbox, one file per folder and month. Every message is stored byte for byte, exactly as it arrived, and closed months never change, so they work well with versioning and Object Lock if you keep your backup in a bucket. Next to the mbox files it writes a manifest.json, a copy of the SQLite index and a README explaining how to open everything without the app.

It works the other way, too: Arca imports .mbox files from Google Takeout, Apple Mail and Thunderbird for free, as an offline account you can search and, if you like, restore into your connected account. More in the FAQ, and if you don’t have a Gmail backup yet, start with this guide.

Arca for Windows is in the Microsoft Store, free for one account. For Mac (macOS 14 or later), it’s in the Mac App Store.