MBOX
A plain-text file format that stores multiple email messages concatenated together, each beginning with a "From " separator line. It is the format Google Takeout produces when you export your Gmail archive.
MBOX (short for mailbox) is one of the oldest and most widely supported email storage formats. Every message is appended one after another in a single text file, with each message separated from the next by a line that starts with "From " (note the trailing space) followed by the sender address and a timestamp. Because the entire archive lives in a single file, MBOX is easy to copy, compress, and store as a long-term backup.
Google Takeout produces MBOX files when you export Gmail data, sometimes splitting very large mailboxes into multiple files. Other services and desktop clients — Apple Mail, Thunderbird, Postfix — also use MBOX variants (mboxo, mboxrd, mboxcl2), which differ mainly in how they escape embedded "From " lines within message bodies.
Opening a large MBOX file without a specialized tool can be slow because most programs parse the entire file before showing any messages. Mbox Viewer addresses this with a streaming parser that reads the file incrementally and a binary index that enables subsequent opens in under a second, regardless of file size.
Related terms
A single-message file in MIME format, containing headers, body, and attachments. EML files are widely supported across email clients and are useful for archiving or sharing individual messages.
A mailbox format that stores each email message as a separate file within a directory hierarchy, rather than concatenating all messages into a single file like MBOX.
Google's official service for exporting your personal data, including Gmail. For email, it produces one or more MBOX files containing all your messages and their Gmail labels.