The application/mbox Media Type
RFC 4155

Network Working Group                                            E. Hall
Request for Comments: 4155                                September 2005
Category: Informational

                    The application/mbox Media Type

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This memo requests that the application/mbox media type be authorized
   for allocation by the IESG, according to the terms specified in RFC
   2048.  This memo also defines a default format for the mbox database,
   which must be supported by all conformant implementations.

1.  Background and Overview

   UNIX-like operating systems have historically made widespread use of
   "mbox" database files for a variety of local email purposes.  In the
   common case, mbox files store linear sequences of one or more
   electronic mail messages, with local email clients treating the
   database as a logical folder of email messages.  mbox databases are
   also used by a variety of other messaging tools, such as mailing list
   management programs, archiving and filtering utilities, messaging
   servers, and other related applications.  In recent years, mbox
   databases have also become common on a large number of non-UNIX
   computing platforms, for similar kinds of purposes.

   The increased pervasiveness of these files has led to an increased
   demand for a standardized, network-wide interchange of these files as
   discrete database objects.  In turn, this dictates a need for a
   general media type definition for mbox files, which is the subject
   and purpose of this memo.

Hall                         Informational                      [Page 1]
RFC 4155            The application/mbox Media Type       September 2005

2.  About the mbox Database

   The mbox database format is not documented in an authoritative
   specification, but instead exists as a well-known output format that
   is anecdotally documented, or which is only authoritatively
   documented for a specific platform or tool.

   mbox databases typically contain a linear sequence of electronic mail
   messages.  Each message begins with a separator line that identifies
   the message sender, and also identifies the date and time at which
   the message was received by the final recipient (either the last-hop
   system in the transfer path, or the system which serves as the
   recipient's mailstore).  Each message is typically terminated by an
   empty line.  The end of the database is usually recognized by either
   the absence of any additional data, or by the presence of an explicit
   end-of-file marker.

   The structure of the separator lines vary across implementations, but
   usually contain the exact character sequence of "From", followed by a
   single Space character (0x20), an email address of some kind, another
   Space character, a timestamp sequence of some kind, and an end-of-
   line marker.  However, due to the lack of any authoritative
   specification, each of these attributes are known to vary widely
   across implementations.  For example, the email address can reflect
   any addressing syntax that has ever been used on any messaging system
   in all of history (specifically including address forms that are not
   compatible with Internet messages, as defined by RFC 2822 [RFC2822]).
   Similarly, the timestamp sequences can also vary according to system
   output, while the end-of-line sequences will often reflect platform-
   specific requirements.  Different data formats can even appear within
   a single database as a result of multiple mbox files being
   concatenated together, or because a single file was accessed by
   multiple messaging clients, each of which has used its own syntax for
   the separator line.

   Message data within mbox databases often reflects site-specific
   peculiarities.  For example, it is entirely possible for the message
   body or headers in an mbox database to contain untagged eight-bit
   character data that implicitly reflects a site-specific default
   language or locale, or that reflects local defaults for timestamps
   and email addresses; none of this data is widely portable beyond the
   local scope.  Similarly, message data can also contain unencoded
   eight-bit binary data, or can use encoding formats that represent a
   specific platform (e.g., BINHEX or UUENCODE sequences).

Hall                         Informational                      [Page 2]
RFC 4155            The application/mbox Media Type       September 2005

   Many implementations are also known to escape message body lines that