.MBOX

Introduction

An mbox file is a whole mail folder in one text file: every message, one after another, each in the same form as an EML file. It is the oldest way of storing mail on Unix systems and is still widely used to move mail between programs. Google Takeout exports Gmail as .mbox, Apple Mail exports each mailbox as an .mbox folder with an mbox file inside, and Thunderbird keeps each of its folders as an mbox file with no extension.

Structure

Each message starts with a separator line that begins From (the word, then a space, with no colon), followed by the sender’s address and the date, for example From alice@example.com Thu Jan 1 00:00:00 2026. The message’s header fields and body follow, and a blank line ends it before the next separator.

Because a line starting From inside a body would look like a new message, programs escape such lines by adding > in front. There are several versions of the rules: mboxo escapes only From lines, mboxrd also escapes lines that already start with >From so the change can be undone, and mboxcl and mboxcl2 add a Content-Length field to give each message’s size instead. Files do not say which version they use, so readers guess.

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