What Is the MBOX Format? A Practical Reference

If you've just exported your Gmail with Google Takeout, dragged a folder out of Thunderbird, or inherited a dusty backup from a Unix server, you're looking at an MBOX file. The format is older than most people working with it, it has incompatible variants that quietly corrupt messages, and almost no modern webmail can import it directly. This page covers what MBOX actually is, where it shows up in migrations, and the gotchas worth knowing before you trust it with a year of someone's email.

The one-line definition#

MBOX is a mailbox file that stores many email messages concatenated into a single plain-text file. Each message starts with a line that begins From (the word "From" followed by a space, no colon), and that line is what tools use to find where one message ends and the next begins.

That's it. There's no database, no index, no per-message file. Open an MBOX in a text editor and you'll see RFC 822 headers, blank line, body, and then another From separator, repeated for every message in the folder.

A short history#

MBOX comes from Version 7 Unix in the late 1970s. The original /var/mail/<username> file on a Unix system was literally an MBOX. When desktop mail clients arrived, most of them kept the same on-disk shape because it was simple, append-only, and easy to back up.

The trade-off: the format was never standardised cleanly. Different mail clients made different choices about how to escape lines inside a message body that happen to start with From . That gave us the variants.

The MBOX variants#

There are four common variants, and they are not always interchangeable.

mboxo#

The original. Lines starting with From inside a message body get prefixed with > on the way in. The escape isn't reversible, so the body you read back isn't byte-identical to the body that arrived. This is the one to avoid for archival use.

mboxrd#

The "reversible" fix. Any line starting with one or more > characters followed by From gets an extra > added. On the way out, you strip one >. Round-trips cleanly. This is what most Unix mail tools, Thunderbird, and Apple Mail use today.

mboxcl / mboxcl2#

Add a Content-Length: header to each message so the parser can skip ahead by byte count instead of scanning for the next From line. Faster on huge files. Rare in the wild.

Where MBOX shows up in migrations#

Three places, almost always.

Google Takeout exports. When you export a Gmail account through takeout.google.com, you get one .mbox file per account (sometimes split into multiple files if the mailbox is large). This is the most common modern source of MBOX in 2026.

Thunderbird local folders. Thunderbird stores every folder as an MBOX file under your profile directory, with a companion .msf index file. When someone retires a Thunderbird workstation, the MBOX files are the only thing worth keeping.

Apple Mail archives. Apple Mail's "Export Mailbox" feature produces .mbox packages — technically a folder with an mbox file inside, plus index files. Still MBOX at the core.

You'll also see MBOX from mutt, Pine, and any IMAP backup tool that talks to a Unix-style mail spool. For the full file-format landscape, see our PST, MBOX, and EML migration guide.

MBOX vs the alternatives#

MBOX is one of three formats you'll meet in any real migration. The others are the PST file (Outlook's single-file archive), and the EML format (one file per message). MBOX sits in between — multiple messages, single file, plain text.

For ongoing mail storage you'll also hear about OST files, which are Outlook's offline cache. OST is not portable. MBOX is.

Converting MBOX to something else#

The honest answer: there's no clean conversion to PST without a tool. The data shape is too different. What works in practice:

1. Parse the MBOX message-by-message.
2. Connect over IMAP to the destination (Gmail, Microsoft 365, Fastmail, whatever).
3. APPEND each message into the right folder, preserving the original Date: header and read/unread state.

That's the path Mailbox Taxi takes — it reads MBOX locally on your machine, never uploads the file to a server it doesn't control, and writes messages to the destination over standard IMAP. If you need provider-specific walkthroughs, see migrate MBOX to Gmail, migrate MBOX to Office 365, or migrate MBOX to Outlook.

If you're coming off Thunderbird specifically, the Thunderbird to Outlook guide covers the profile-folder layout in more detail.