---
title: "mboxrd / mboxo — Glossary — Mbox Viewer"
description: "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."
url: https://mboxviewerpro.com/glossary/mboxrd/
language: en
source: Mbox Viewer
---

[Mbox Viewer](https://mboxviewerpro.com/)  [Glossary](https://mboxviewerpro.com/glossary/)  mboxrd / mboxo

# mboxrd / mboxo

From-quoting · mboxcl2

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.

In an MBOX file, a line starting with "From " marks the beginning of a new message. That creates an obvious problem: what if a message body contains such a line, in a quoted reply or a code sample? The variants of the format are the different answers to that question. mboxo prefixes the offending line with a > character. mboxrd does the same but also escapes lines that already begin with >From, adding another >, which makes the transformation reversible — you can always recover the original text. mboxcl and mboxcl2 take another route entirely, recording each message's length in a Content-Length header.

This matters more than it sounds, because not every program that ever wrote your mailbox followed the rules. A file may well contain an unescaped body line starting with "From ". Any tool that splits an archive by searching for that pattern will tear such a message in two, turning one email into two half-emails without warning.

It is the main reason to treat mbox files as mail rather than as text: parse them, select whole messages, and check that the message count before and after a split still matches. Mbox Viewer and mboxShell read every variant of the family, so you do not need to know which one produced your file.

## Related terms

[

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.

](https://mboxviewerpro.com/glossary/mbox/)[

Envelope

The delivery metadata used by SMTP servers to route an email message — specifically the envelope sender (MAIL FROM) and envelope recipients (RCPT TO) — which may differ from the visible From and To headers.

](https://mboxviewerpro.com/glossary/envelope/)[

MBX (Eudora)

Eudora's mailbox file: plain mbox under a different extension, paired with a .toc index file. Readable today with no conversion — but its attachments were stored outside the mailbox.

](https://mboxviewerpro.com/glossary/mbx/)

[Back to the full glossary](https://mboxviewerpro.com/glossary/)

## Read your MBOX files on Mac and Windows

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