mboxrd / mboxo

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.