An mbox file is a plain-text file that stores multiple email messages back-to-back in a single document, separated by lines that begin with the five characters From (capital F, lowercase rest, trailing space). The format is defined by RFC 4155 and has been the default email archive format on Unix-family systems since the 1970s.
Why mbox still matters in 2026
Any time an email archive has to leave its original mail app, mbox is the lingua franca. Gmail Takeout, Thunderbird exports, macOS Mail archives, Mutt, and most e-discovery exports all produce mbox. The format is plain text, so a .mbox file opened 30 years from now will still be readable — unlike proprietary formats that break with software upgrades.
What’s inside an mbox file
Every message is stored sequentially:
- A separator line starting with
Fromplus sender and timestamp - Standard email headers (
From:,To:,Subject:,Date:,Message-ID:, etc.) - A blank line
- The message body, in the original MIME encoding
Attachments are embedded inline as MIME parts — the mbox doesn’t reference external files. A single .mbox from a heavy mailbox can exceed 50 GB.
The four mbox variants
| Variant | What’s special | Who uses it |
|---|---|---|
| mboxo | Original Unix variant | Legacy Unix |
| mboxrd | Escapes literal From in bodies with > | Gmail Takeout, Thunderbird |
| mboxcl | Uses Content-Length: header | Some SunOS mail |
| mboxcl2 | Like mboxcl, no escaping | Rare |
In practice, modern tools produce mboxrd. Any competent mbox reader auto-detects the variant.
Tools that read mbox
- Thunderbird — via “Import Local Folders”
- macOS Mail — via File → Import Mailboxes
- Mutt —
mutt -f archive.mbox - Python — built-in
mailboxmodule - tomorrow-box — a modern GUI reader built for today’s hardware
When mbox is the wrong choice
Mbox is a read-mostly archive format. If you need random-access message updates (marking read, moving between folders, concurrent writes), use maildir or a server-side IMAP account instead. Mbox shines for archival, forensic work, and one-shot reads.
TL;DR
Mbox = every message in one plain-text file, separated by From lines. If you exported email from almost anywhere, you already have one.