MBOX
.mbox · mbox file · mailbox fileA 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.
One last thing, because it sends people to the wrong place: the name collides with things that have nothing to do with email. Makita's M-Box is a stackable tool case, and the Ford Thunderbird is a car — while Thunderbird, the Mozilla email client, does store its mail in MBOX files. If you got here after downloading something ending in .mbox from Google, Apple Mail or Thunderbird, you are in the right place: it is a mailbox, and it is plain text.
Step-by-step guides
- How to Open an MBOX File on Mac (Even Huge Ones)
Three ways to open an MBOX file on macOS — Apple Mail, Thunderbird or a dedicated viewer — with the steps for each, and which one to pick for a multi-gigabyte Gmail archive.
- How to Open an MBOX File on Windows 10 and 11
Three ways to open and read an MBOX file on Windows: import it into Thunderbird, open it with a dedicated viewer, or take a quick look. Steps for each, and which one fits your file.
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.
The MBOX family's variants, which differ in how they protect body lines that begin with "From ". mboxrd escapes them reversibly; mboxo does not; mboxcl and mboxcl2 use a Content-Length header instead.