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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).


   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.

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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).

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RFC 4155            The application/mbox Media Type       September 2005

   Many implementations are also known to escape message body lines that
   begin with the character sequence of "From ", so as to prevent
   confusion with overly-liberal parsers that do not search for full
   separator lines.  In the common case, a leading Greater-Than symbol
   (0x3E) is used for this purpose (with "From " becoming ">From ").
   However, other implementations are known not to escape such lines
   unless they are immediately preceded by a blank line or if they also
   appear to contain an email address and a timestamp.  Other
   implementations are also known to perform secondary escapes against
   these lines if they are already escaped or quoted, while others
   ignore these mechanisms altogether.

   A comprehensive description of mbox database files on UNIX-like
   systems can be found at http://qmail.org./man/man5/mbox.html, which
   should be treated as mostly authoritative for those variations that
   are otherwise only documented in anecdotal form.  However, readers
   are advised that many other platforms and tools make use of mbox
   databases, and that there are many more potential variations that can
   be encountered in the wild.

   In order to mitigate errors that may arise from such vagaries, this
   specification defines a "format" parameter to the application/mbox
   media type declaration, which can be used to identify the specific
   kind of mbox database that is being transferred.  Furthermore, this
   specification defines a "default" database format which MUST be
   supported by implementations that claim to be compliant with this
   specification, and which is to be used as the implicit format for
   undeclared application/mbox data objects.  Additional format types
   are to be defined in subsequent specifications.  Messaging systems
   that receive an mbox database with an unknown format parameter value
   SHOULD treat the data as an opaque binary object, as if the data had
   been declared as application/octet-stream

   Refer to Appendix A for a description of the default mbox format.

   Note that RFC 2046 [RFC2046] defines the multipart/digest media type
   for transferring platform-independent message files.  Because that
   specification defines a set of neutral and strict formatting rules,
   the multipart/digest media type already facilitates highly-
   predictable transfer and conversion operations; as such, implementers
   are strongly encouraged to support and use that media type where

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3.  Prerequisites and Terminology

   Readers of this document are expected to be familiar with the
   specification for MIME [RFC2045] and MIME-type registrations

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

4.  The application/mbox Media Type Registration

   This section provides the media type registration application (as per

   MIME media type name: application

   MIME subtype name: mbox

   Required parameters: none

   Optional parameters: The "format" parameter identifies the format of
   the mbox database and the messages contained therein.  The default
   value for the "format" parameter is "default", and refers to the
   formatting rules defined in Appendix A of this memo.  mbox databases
   that do not have a "format" parameter SHOULD be interpreted as having
   the implicit "format" value of "default".  mbox databases that have
   an unknown value for the "format" parameter SHOULD be treated as
   opaque data objects, as if the media type had been specified as
   application/octet-stream.  Additional values for the format parameter
   are to be defined in subsequent specifications, and registered with

   Encoding considerations: If an email client receives an mbox database
   as a message attachment, and then stores that attachment within a
   local mbox database, the contents of the two database files may
   become irreversibly intermingled, such that both databases are
   rendered unrecognizable.  In order to avoid these collisions,
   messaging systems that support this specification MUST encode an mbox
   database (or at a minimum, the separator lines) with non-transparent
   transfer encoding (such as BASE64 or Quoted-Printable) whenever an
   application/mbox object is transferred via messaging protocols.
   Other transfer services are generally encouraged to adopt similar
   encoding strategies in order to allow for any subsequent
   retransmission that might occur, but this is not a requirement.
   Implementers should also be prepared to encode mbox data locally if
   non-compliant data is received.

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   Security considerations: mbox data is passive, and does not generally
   represent a unique or new security threat.  However, there is risk in
   sharing any kind of data, because unintentional information may be
   exposed, and this risk certainly applies to mbox data as well.

   Interoperability considerations: Due to the lack of a single
   authoritative specification for mbox databases, there are a large
   number of variations between database formats (refer to the
   introduction text for common examples), and it is expected that non-
   conformant data will be erroneously tagged or exchanged.  Although
   the "default" format specified in this memo does not allow for these
   kinds of vagaries, prior negotiation or agreement between humans may
   sometimes be needed.

   Published specification: see Appendix A.

   Applications that use this media type: hundreds of messaging products
   make use of the mbox database format, in one form or another.

   Magic number(s): mbox database files can be recognized by having a
   leading character sequence of "From", followed by a single Space
   character (0x20), followed by additional printable character data
   (refer to the description in Appendix A for details).  However,
   implementers are cautioned that all such files will not be compliant
   with all of the formatting rules, therefore implementers should treat
   these files with an appropriate amount of circumspection.

   File extension(s): mbox database files sometimes have an ".mbox"
   extension, but this is not required nor expected.  As with magic
   numbers, implementers should avoid reflexive assumptions about the
   contents of such files.

   Macintosh File Type Code(s): None are known to be common.

   Person & email address to contact for further information: Eric A.
   Hall (ehall@ntrg.com)

   Intended usage: COMMON

5.  Security Considerations

   See the discussion in section 4.

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6.  IANA Considerations

   The IANA has registered the application/mbox media type in the MIME
   registry, using the application provided in section 4 above.

   Furthermore, IANA has established and will maintain a registry of
   values for the "format" parameter as described in this memo.  The
   first registration is the "default" value, using the description
   provided in Appendix A.  Subsequent values for the "format" parameter
   MUST be accompanied by some form of recognizable, complete, and
   legitimate specification, such as an IESG-approved specification, or
   some kind of authoritative vendor documentation.

7.  Normative References

   [RFC2045]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part One: Format of Internet Message
               Bodies", RFC 2045, November 1996.

   [RFC2046]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part Two: Media Types", RFC 2046,
               November 1996.

   [RFC2048]   Freed, N., Klensin, J., and J. Postel, "Multipurpose
               Internet Mail Extensions (MIME) Part Four: Registration
               Procedures", BCP 13, RFC 2048, November 1996.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2822]   Resnick, P., "Internet Message Format", RFC 2822, April

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Appendix A.  The "default" mbox Database Format

   In order to improve interoperability among messaging systems, this
   memo defines a "default" mbox database format, which MUST be
   supported by all implementations that claim to be compliant with this

   The "default" mbox database format uses a linear sequence of Internet
   messages, with each message being immediately prefaced by a separator
   line, and being terminated by an empty line.  More specifically:

      o Each message within the database MUST follow the syntax and
        formatting rules defined in RFC 2822 [RFC2822] and its related
        specifications, with the exception that the canonical mbox
        database MUST use a single Line-Feed character (0x0A) as the
        end-of-line sequence, and MUST NOT use a Carriage-Return/Line-
        Feed pair (NB: this requirement only applies to the canonical
        mbox database as transferred, and does not override any other
        specifications).  This usage represents the most common
        historical representation of the mbox database format, and
        allows for the least amount of conversion.

      o Messages within the default mbox database MUST consist of
        seven-bit characters within an eight-bit stream.  Eight-bit data
        within the stream MUST be converted to a seven-bit form (using
        appropriate, standardized encoding) and appropriately tagged
        (with the correct header fields) before the database is

      o Message headers and data in the default mbox database MUST be
        fully-qualified, as per the relevant specification(s).  For
        example, email addresses in the various header fields MUST have
        legitimate domain names (as per RFC 2822), while extended
        characters and encodings MUST be specified in the appropriate
        location (as per the appropriate MIME specifications), and so

      o Each message in the mbox database MUST be immediately preceded
        by a single separator line, which MUST conform to the following

           The exact character sequence of "From";

           a single Space character (0x20);

           the email address of the message sender (as obtained from the
           message envelope or other authoritative source), conformant
           with the "addr-spec" syntax from RFC 2822;

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           a single Space character;

           a timestamp indicating the UTC date and time when the message
           was originally received, conformant with the syntax of the
           traditional UNIX 'ctime' output sans timezone (note that the
           use of UTC precludes the need for a timezone indicator);

           an end-of-line marker.

      o Each message in the database MUST be terminated by an empty
        line, containing a single end-of-line marker.

   Note that the first message in an mbox database will only be prefaced
   by a separator line, while every other message will begin with two
   end-of-line sequences (one at the end of the message itself, and
   another to mark the end of the message within the mbox database file
   stream) and a separator line (marking the new message).  The end of
   the database is implicitly reached when no more message data or
   separator lines are found.

   Also note that this specification does not prescribe any escape
   syntax for message body lines that begin with the character sequence
   of "From ".  Recipient systems are expected to parse full separator
   lines as they are documented above.

Author's Address

   Eric A. Hall

   EMail: ehall@ntrg.com

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