What Is an MBOX File?

Definition

An MBOX file is a plain-text container that holds multiple email messages in a single file. Each message is a full RFC 5322 email — headers, blank line, body — and messages are separated by a line beginning with From  (the word From followed by a space). The file extension is .mbox, though many Unix tools store mailboxes without any extension.

The format is vendor-neutral, uncompressed, and human-readable. That is both its strength (any text tool can process it) and its weakness (it is not optimized for size or random access).

Anatomy of an MBOX file

An MBOX file looks like a concatenation of email messages. Here is a minimal example with two messages:

From [email protected] Mon Apr 22 09:12:00 2026
From: [email protected]
To: [email protected]
Subject: First message
Date: Mon, 22 Apr 2026 09:12:00 +0000

Hello Bob, this is the first message.

From [email protected] Mon Apr 22 09:13:00 2026
From: [email protected]
To: [email protected]
Subject: Second message
Date: Mon, 22 Apr 2026 09:13:00 +0000

Hello again.

The key element is the separator line — any line that starts with From  (note the trailing space, sometimes called From_) marks the start of a new message. This is different from the From: header inside the message, which has a colon.

This simple rule is also the format's most famous flaw: if a message body contains a line that happens to start with From , a naive parser will split the message in two. The variants below exist mainly to solve this.

MBOX format variants

Over the decades, four MBOX variants emerged:

• mboxo — the original Unix format. No escaping. Ambiguous if bodies contain From  at the start of a line.
• mboxrd — the modern default. Escapes From  in bodies by prefixing it with > (so From you becomes >From you). Used by Mozilla Thunderbird and most modern tools.
• mboxcl — adds a Content-Length header so parsers can read a fixed number of bytes instead of scanning for From . Used historically by SCO and some SVR4 Unixes.
• mboxcl2 — like mboxcl but does not escape From  lines. Obsolete.

If you are working with MBOX files from Apple Mail, Thunderbird, or Gmail Takeout in 2026, you are almost certainly dealing with mboxrd.

Which applications use MBOX

MBOX is the storage format for:

• Apple Mail — default local storage on macOS. Export via Mailbox > Export Mailbox.
• Mozilla Thunderbird — default local storage (mboxrd variant). Files live in your profile folder, typically under ~/Library/Thunderbird/Profiles/*/Mail/ on macOS.
• Gmail (via Google Takeout) — Google exports your Gmail archive as a single .mbox file.
• Proton Mail (via the Proton Mail Export Tool) — exports a per-label .mbox file.
• Eudora — legacy mail client, still a common source for old archives.
• Evolution, KMail, Alpine, mutt, Pine — Unix/Linux mail clients, native MBOX support.

Microsoft Outlook does not use MBOX. Outlook uses the proprietary PST format; converting Outlook email to MBOX usually requires a third-party tool or an intermediate export to EML.

MBOX vs EML vs PST

Three formats frequently come up when working with email archives:

How to open an MBOX file on Mac

You have three practical options on macOS:

1. Re-import into a mail client. Apple Mail (File > Import Mailboxes) and Thunderbird can read MBOX directly. Good if you want to read messages one by one.
2. Convert to a searchable format. If you want to search, filter, or analyze at scale, convert the MBOX to CSV or Excel. MBOX to CSV Data Extractor is a native macOS app that does this 100% on-device, with no uploads.
3. Open in a text editor. TextEdit or VS Code will show the raw file. You will see message headers and bodies, but HTML parts look like source code and attachments appear as Base64-encoded blobs — not readable.

Why convert MBOX to CSV

Storing email in MBOX is fine. Analyzing email in MBOX is not. Converting to CSV, Excel, JSON, or XML unlocks:

• E-discovery and legal hold — tabulated sender/recipient/date/subject columns that can be sorted, filtered, and exported to review platforms.
• Migration planning — counts by sender, date range, attachment size.
• Research corpora — feeding email text into NLP pipelines, sentiment analysis, or academic datasets.
• Compliance — producing a machine-readable export of a specific mailbox for a given date range.
• Personal archiving — a spreadsheet of "every email I sent in 2025" is just easier to browse than a mailbox.

FAQ

Related

• MBOX to CSV vs online MBOX converters →
• How to convert MBOX to CSV on Mac →
• MBOX to CSV FAQ →
• Convert MBOX to PDF instead →