View source for Mbox

== MBOX family ==
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The mbox format uses a single blank line followed by the string 'From ' (with a space) to delimit messages; this can create ambiguities if a message contains the same sequence in the message text.

Over the years, four popular but incompatible variants arose: ''mboxo'', ''mboxrd'', ''mboxcl'', and ''mboxcl2''. The naming scheme was developed by [[Daniel J. Bernstein]], Rahul Dhesi, and others in 1996. Each originated from a different version of [[Unix]]. ''mboxcl'' and ''mboxcl2'' originated from the file format used by Unix System V Release 4 mail tools. ''mboxrd'' was invented by Rahul Dhesi et al. as a rationalization of ''mboxo'' and subsequently adopted by some Unix mail tools including [[qmail]].

All these variants have the problem that the content of the message sometimes must be modified to remove ambiguities, as shown below, so that applications have to know which quoting rule has been used to perform the correct reversion, which turned out to be impractical. Using [[MIME]] and choosing a content-transfer-encoding that quotes "From_" lines in a standard-compliant fashion ensures that message content doesn't need to be changed, but only their [[MIME]] representation. Therefore, checksums remain constant, a necessary precondition for supporting [[S/MIME]] and [[Pretty Good Privacy]]. Applications that newly create messages and store them in mbox database files will likely use this approach to detach message content from database storage format.

''mboxo'' and ''mboxrd'' locate the message start by scanning for ''From '' lines that are found before the email message headers.  If a "<code>From&nbsp;</code>" string occurs at the beginning of a line in either the header or the body of a message (a mail standard violation for the former, but not for the latter), the email message must be modified before the message is stored in an '''mbox''' mailbox file or the line will be taken as a message boundary. To avoid misinterpreting a "<code>From&nbsp;</code>" string at the beginning of the line in the email body as the beginning of a new email, some systems '''"From-munge"'''<ref name="RFC3676Section4.4">{{cite web |editor-last=Gellens |editor-first=R. |title=Request for Comments: 3676 – The Text/Plain Format and DelSp Parameters – Section 4.4: Space-Stuffing |url=https://datatracker.ietf.org/doc/html/rfc3676#section-4.4 |date=February 2004 |website=[[Internet Engineering Task Force]] |access-date=17 May 2021 |archive-date=16 May 2021 |archive-url=https://web.archive.org/web/20210516191246/https://datatracker.ietf.org/doc/html/rfc3676#section-4.4 |url-status=live }}</ref> the message, typically by prepending a greater-than sign:
    >From my point of view...

In the ''mboxo'' format, such lines have irreversible ambiguity.<ref>[http://www.jwz.org/doc/content-length.html "Configuring Netscape Mail On Unix: Why the Content-Length Format is Bad"] {{Webarchive|url=https://web.archive.org/web/20090408050212/http://www.jwz.org/doc/content-length.html |date=2009-04-08 }}
by Jamie Zawinski 1997</ref>
In the ''mboxo'' format, this can lead to corruption of the message.  If a line already contained <code>>From&nbsp;</code> at the beginning (such as in a quotation), it is unchanged when written. When subsequently read by the mail software, the leading <code>></code> is erroneously removed.  The ''mboxrd'' format solves this by converting <code>From&nbsp;</code> to <code>>From&nbsp;</code> and converting <code>>From&nbsp;</code> to <code>>>From&nbsp;</code>, etc.  The transformation is then always reversible.<ref>{{cite web |author-first=Jonathan |author-last=de Boyne Pollard |title="mbox" is a family of several mutually incompatible mailbox formats |url=http://jdebp.eu./FGA/mail-mbox-formats.html |year=2004 |archive-url=https://web.archive.org/web/20201231033049/http://jdebp.eu./FGA/mail-mbox-formats.html |archive-date=31 December 2020 |url-status=dead |department=Frequently Given Answers |access-date=20 March 2023}}</ref>

Example:
<syntaxhighlight lang="email">
From MAILER-DAEMON Fri Jul  8 12:08:34 2011
From: Author <author@example.com>
To: Recipient <recipient@example.com>
Subject: Sample message 1
 
This is the body.
>From (should be escaped).
There are 3 lines.
 
From MAILER-DAEMON Fri Jul  8 12:08:34 2011
From: Author <author@example.com>
To: Recipient <recipient@example.com>
Subject: Sample message 2
 
This is the second body.
</syntaxhighlight>
The ''mboxcl'' and ''mboxcl2'' formats use a <code>Content-Length:</code> header to determine the messages' lengths and thereby the next ''real From&nbsp;line''. ''mboxcl'' still quotes ''From&nbsp;&nbsp;lines'' in the messages themselves as ''mboxrd'' does, while ''mboxcl2'' doesn't.

=== Modified mbox ===
Some [[email client]]s use a modification of the mbox format for their mail folders.
* [[Eudora (email client)|Eudora]] used an ''mboxo'' variation where a sender's email address is replaced by the constant string "???@???".  Most mbox clients store incoming messages as received.  Eudora separates out attachments embedded in the message, storing the attachments as separate individual files in one folder.<ref>{{Cite web|url = http://www.eudora.com/download/eudora/mac/6.2.4/Eudora_6.2.4_Mac_User_Guide.pdf|title = Eudora 6.2.4 Mac User Guide|access-date = 2015-10-29|url-status = dead|archive-url = https://web.archive.org/web/20140712122540/http://www.eudora.com/download/eudora/mac/6.2.4/Eudora_6.2.4_Mac_User_Guide.pdf|archive-date = 2014-07-12|page = 113}}</ref>
* The [[Mozilla]] family of email clients (Mozilla, Netscape, Thunderbird, et al.) use an ''mboxrd'' variation with more complex ''From line'' quoting rules.<ref>{{Cite web|url=http://kb.mozillazine.org/Importing_and_exporting_your_mail#Mbox_files|title=Importing and exporting your mail - MozillaZine Knowledge Base|website=kb.mozillazine.org|access-date=2011-06-18|archive-date=2013-07-03|archive-url=https://web.archive.org/web/20130703053056/http://kb.mozillazine.org/Importing_and_exporting_your_mail#Mbox_files|url-status=live}}</ref>