Network Working Group               N. Borenstein, Bellcore
            Request for Comments: 1344                        June 1992
                  Implications of MIME for Internet Mail Gateways
          Status of This Memo
            This is an informational memo for  the  Internet  community,
            and  requests  discussion  and suggestions for improvements.
            This  memo  does   not   specify   an   Internet   standard.
            Distribution of this memo is unlimited.
            The recent development of MIME (Multipurpose  Internet  Mail
            Extensions)  offers  a  wide  range of new opportunities for
            electronic mail system systems.  Most of these  opportunites
            are relevant only to user agents, the programs that interact
            with human users when they send and receive mail.   However,
            some  opportunities  are  also  opened up for mail transport
            systems.  While MIME was carefully designed so that it  does
            not  require  any  changes  to  Internet  electronic message
            transport  facilities,  there  are  several  ways  in  which
            message  transport  systems  may  want  to take advantage of
            MIME.  These opportunities are the subject of this memo.
          Background -- The MIME Format
            Recently, a new standardized format  has  been  defined  for
            enhanced  electronic  mail  messages  on the Internet.  This
            format, known as MIME, permits messages  to  include,  in  a
            standardized  manner,  non-ASCII  text, images, audio, and a
            variety of other kinds of interesting data.
            The  MIME  effort  was  explicitly  focused   on   requiring
            absolutely  no  changes  at  the  message  transport  level.
            Because of this fact, MIME-format mail runs transparently on
            all  known  Internet  or  Internet-style mail systems.  This
            means that those concerned solely with the  maintenance  and
            development  of message transport services can safely ignore
            MIME completely, if they so choose.
            However, the fact that MIME can be ignored, for the  purpose
            of  message  transport,  does  not  necessarily mean that it
            should be  ignored.   In  particular,  MIME  offers  several
            features that should be of interest to those responsible for
            message transport services. By  exploiting  these  features,
            transport  systems  can  provide certain additional kinds of
            service that are currently unavailable, and can alleviate  a
            few existing problems.
            The remainder of this document  is  an  attempt  to  briefly
            point  out  and  summarize some important ways in which MIME
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            RFC 1344           MIME and Mail Gateways          June 1992
            may be of use for message transport systems.  This  document
            makes no attempt to present a complete technical description
            of MIME, however.  For that, the reader is  refered  to  the
            MIME document itself [RFC-1341].
          Mail Transport and Gateway Services:  A Key Distinction
            Before implementing any of the mechanisms discussed in  this
            memo,  one  should  be familiar with the distinction between
            mail transport service and mail gateway service.  Basically,
            mail  transport software is responsible for moving a message
            within a homogeneous electronic mail service network.   Mail
            gateways,  on  the  other  hand,  exchange  mail between two
            significantly different  mail  environments,  including  via
            non-electronic services, such as postal mail.
            In general, it is widely considered  unacceptable  for  mail
            transport  services  to  alter the contents of messages.  In
            the case of mail gateways, however, such alteration is often
            inevitable.  Thus, strictly speaking, many of the mechanisms
            described here apply only to gateways,  and  should  not  be
            used  in  simple  mail  transport  systems.   However, it is
            possible that some very special situations -- e.g., an  SMTP
            relay   that  transports  mail  across  extremely  expensive
            intercontinental network  links  --  might  need  to  modify
            messages,  in order to provide appropriate service for those
            situations, and hence must redefine its role to be that of a
            In this memo, it is assumed that transformations which alter
            a message's contents will be performed only by gateways, but
            it is recognized that some existing  mail  transport  agents
            may  choose to reclassify themselves as gateways in order to
            perform the functions described here.
          Rejected Messages
            An unfortunately frequent duty of message transport services
            is  the  rejection  of  mail to the sender.  This may happen
            because the mail was undeliverable, or because  it  did  not
            conform  to  the requirements of a gateway (e.g., it was too
            There has never been a standard format for rejected messages
            in  the  past.   This has been an annoyance, but not a major
            problem for text messages.  For non-text messages,  however,
            the  lack  of  a  standard rejection format is more crucial,
            because rejected messages typically appear to be  text,  and
            the  user  who  finds  himself viewing images or audio as if
            they were text is rarely happy with the result.
            MIME makes it very easy to encapsulate messages  in  such  a
            way  that  their  semantics  are  completely preserved.  The
            simplest way to do this is to make each rejection  notice  a
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            RFC 1344           MIME and Mail Gateways          June 1992
            MIME  "multipart/mixed"  message.   That  multipart  message
            would contain two parts, a text part explaining  the  reason
            for  the  rejection,  and  an encapsulated message part that
            contained the rejected message itself.
            It should be stressed that the transport software  does  not
            need  to understand the structure of the rejected message at
            all.  It  merely  needs  to  encapsulate  it  properly.  The
            following,  for  example,  shows how any MIME message may be
            encapsulated in a rejection message in such a way  that  all
            information  will be immediately visible in the correct form
            if the  recipient  reads  it  with  a  MIME-conformant  mail
                 From: Mailer-Daemon <>
                 Subject: Rejected Message
                 Content-type: multipart/mixed; boundary=unique-boundary
                 Content-type: text/plain; charset=us-ascii
                 A mail message you sent was rejected.  The details of
                 the rejected message are as follows:
                 From: Nathainel Borenstein <>
                 Message-ID: <>
                 Subject: I know my rights!
                 Rejection-reason:  No mail from libertarians is
                 The original message follows below.
                 Content-type: message/rfc822
                 The ENTIRE REJECTED MESSAGE, starting with the headers,
                 goes here.
            In  the  above  example,  the  ONLY  thing   that   is   not
            'boilerplate"  is the choice of boundary string.  The phrase
            "unique-boundary" should be replaced by a string  that  does
            not  appear  (prefixed  by  two  hyphens) in any of the body
            Encapsulating a message in this manner is very easily  done,
            and  will  constitute  a  significant  service  that message
            transport services can perform for MIME users.
            IMPORTANT NOTE:  The format given above  is  simply  one  of
            many possible ways to format a rejection message using MIME.
            Independent IETF efforts are needed in order to  standardize
            the format of rejections and acknowledgements.
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            RFC 1344           MIME and Mail Gateways          June 1992
          Fragmenting and Reassembling Large Messages
            One  problem  that  occurs  with  increasing  frequency   in
            Internet  mail  is the rejection of messages because of size
            limitations.   This  problem  can  be   expected   to   grow
            substantially  more  severe  with the acceptance of MIME, as
            MIME invites the use of very large objects  such  as  images
            and audio clips.  Fortunately, MIME also provides mechanisms
            that can help alleviate the problem.
            One particularly relevant MIME  type  is  "message/partial",
            which  can  be  used  for  the  automatic  fragmentation and
            reassembly of large mail messages.  The message/partial type
            can be handled entirely at the user agent level, but message
            transport services can also make use of this type to provide
            more intelligent behavior at gateways.
            In particular, when gatewaying mail to or from a  system  or
            network  known  to enforce size limitations that are more or
            less stringent than are enforced locally, message  transport
            services  might  choose either to break a large message into
            fragments, or (perhaps less likely) to reassemble  fragments
            into  a  larger  message.   The  combination  of  these  two
            behaviors can make the  overall  Internet  mail  environment
            appear more complete and seamless than it actually is.
            Details on the message/partial format may be  found  in  the
            MIME  document.   What follows is an example of how a simple
            short message  might  be  broken  into  two  message/partial
            messages.   In  practice,  of  course,  the  message/partial
            facility would only be likely to be  used  for  much  longer
            The following initial message:
                 From:  Nathaniel Borenstein <>
                 To: Ned Freed: <>
                 Subject: a test message
                 Content-type: image/gif
                 Content-Transfer-Encoding: base64
            can  be  transformed,  invertibly,  into  the  following two
            message/partial messages:
                 From:  Nathaniel Borenstein <>
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            RFC 1344           MIME and Mail Gateways          June 1992
                 To: Ned Freed <>
                 Subject: a test message
                 Content-type: message/partial; id="";
                      number=1; total=2
                 Content-type: image/gif
                 Content-Transfer-Encoding: base64
                 From:  Nathaniel Borenstein <>
                 To: Ned Freed <>
                 Subject: a test message
                 Content-type: message/partial; id="";
                      number=2; total=2
            Fragmenting such messages rather than rejecting  them  might
            be  a  reasonable option for some gateway services, at least
            for a certain range of message  sizes.   Of  course,  it  is
            often  difficult for a gateway to know what size limitations
            will  be encountered "downstream",  but intelligent  guesses
            are often possible.  Moreover, an IETF working group on SMTP
            extensions has proposed augmenting SMTP with a  "SIZE"  verb
            that   would   facilitate  this  process,  thereby  possibly
            requiring  fragmentation   on   the   fly   during   message
            Note also that fragmentation or reassembly might  reasonably
            be  performed,  in  differing  circumstances,  by either the
            sending or receiving gateway  systems,  depending  on  which
            system knew more about the capabilities of the other.
          Using or Removing External-Body Pointers
            Another MIME type oriented to extremely  large  messages  is
            the  "message/external-body" type.  In this type of message,
            all or part of the body data is not included in  the  actual
            message  itself.   Instead,  the  Content-Type  header field
            includes information that tells how the  body  data  can  be
            retrieved -- either via a file system, via anonymous ftp, or
            via other mechanisms.
            The message/external-body type provides  a  new  option  for
            mail  transport  services  that  wishes  to optimize the way
            bandwidth resources are used in a  given  environment.   For
            example, the basic use of message/external-body is to reduce
            bandwidth in email traffic. However, when  email  crosses  a
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            RFC 1344           MIME and Mail Gateways          June 1992
            slow and expensive boundary -- e.g., a satellite link across
            the Pacific -- it might make  sense  to  retrieve  the  data
            itself  and  transform  the external-body reference into the
            actual data.  Alternately, it might make sense to  copy  the
            data  itself  to  a  new  location,  closer  to  the message
            recipients, and  change  the  location  pointed  to  in  the
            message.    Because   the  external-body  specification  can
            include an expiration date, message transport  services  can
            trade  off  storage  and  bandwidth  capabilities  to try to
            optimize  the  overall  use  of  resources  for  very  large
            Such behaviors by a  gateway  require  careful  analysis  of
            cost/benefit   tradeoffs  and  would be a dramatic departure
            from  typical  mail  transport   services.    However,   the
            potential  benefits  are quite significant, so that such the
            appropriate use of these service options should be explored.
            For example, the following message includes PostScript  data
            by external reference:
                 From:  Nathaniel Borenstein <>
                 To: Ned Freed <>
                 Subject: The latest MIME draft
                 Content-Type: message/external-body;
                      expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
                 Content-type: application/postscript
            A gateway to Australia might choose to copy the file  to  an
            Australian  FTP archive, changing the relevant parameters on
            the Content-type header field.  Alternately, it might choose
            simply  to  transform  the message into one in which all the
            data were included:
                 From:  Nathaniel Borenstein <>
                 To: Ned Freed <>
                 Subject: The latest MIME draft
                 Content-type: application/postscript
                 %%Creator: greenbush:nsb (Nathaniel Borenstein,MRE 2A-
            This is an example which suggests both the benefits and  the
            dangers.  There  is considerable benefit to having a copy of
            the data immediately  available,   but  there  also  may  be
            considerable  expense involved in transporting it to all  of
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            RFC 1344           MIME and Mail Gateways          June 1992
            a the members of a list, if only a few  will  use  the  data
            anytime soon.
            Alternatively, instead of replacing an external-body message
            with  its real contents, it might make sense to transform it
            into a "multipart/alternative" message containing  both  the
            external  body  reference  and  the  expanded version.  This
            means that only the external body part can be  forwarded  if
            desired,  and  the recipient doesn't lose the information as
            to where the data was fetched from, if they want to fetch an
            up-to-date version in the future.  Such information could be
            represented, in MIME, in the following form:
                 From:  Nathaniel Borenstein <>
                 To: Ned Freed <>
                 Subject: The latest MIME draft
                 Content-type: multipart/alternative; boundary=foo
                 Content-Type: message/external-body;
                      expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
                 Content-type: application/postscript
                 Content-type: application/postscript
                 %%Creator: greenbush:nsb (Nathaniel Borenstein,MRE 2A-
            Similarly for the case where a message is copied to a  local
            FTP  site,  one  could  offer two external body parts as the
            alternatives, allowing the user agent to  choose  which  FTP
            site is preferred.
          Image and other Format Conversions
            MIME currently defines  two  image  formats,  image/gif  and
            image/jpeg.   The  former  is  much more convenient for many
            users, and can be displayed more quickly  on  many  systems.
            The  latter  is  a  much  more  compact  representation, and
            therfore places less stress on mail transport facilities.
            Message  transport  services  can  optimize  both  transport
            bandwidth  and  user  convenience by intelligent translation
            between these formats (and other formats that might be added
            later).   When  a message of type image/gif is submitted for
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            RFC 1344           MIME and Mail Gateways          June 1992
            long-haul delivery, it might  reasonably  be  translated  to
            image/jpeg.   Conversely,  when  image/jpeg data is received
            for  final  delivery  on  a  system  with  adequate  storage
            resources,  it  might  be  translated  to  image/gif for the
            convenience of the recipient.   Software  to  perform  these
            translations  is  widely  available.   It  should  be noted,
            however,  that  performance  of  such  conversions  presumes
            support for the new format by the recipient.
            Although MIME currently only defines one audio format,  more
            are  likely  to  be  defined and registered with IANA in the
            future.  In that case, similar format conversion  facilities
            might be appropriate for audio.
            If format conversion is done,  it  is  STRONGLY  RECOMMENDED
            that some kind of trace information (probably in the form of
            a Received header field) should be added  to  a  message  to
            document the conversion that has been performed.
            Some people have expressed concerns,  or  even  the  opinion
            that  conversions  should  never be done.  To accomodate the
            desires of those who dislike the idea  of  automatic  format
            conversion.   For  this  reason,  it  is suggested that such
            transformations be generally restricted to  gateways  rather
            than  general  message transport services, and that services
            which perform such conversions  should  be  sensitive  to  a
            header field that indicates that the sender does not wish to
            have any such conversions performed.  A suggested value  for
            this header field is:
            Content-Conversion: prohibited
            User agents that wish to explicitly indicate  a  willingness
            for such conversions to be performed may use:
            Content-Conversion: permitted
            However,  this  will  be  the  default  assumption  of  many
            gateways,  so  this  header field is not strictly necessary.
            It also should be noted  that  such  control  of  conversion
            would only be available to the sender, rather than to any of
            the recipients.
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            RFC 1344           MIME and Mail Gateways          June 1992
          Robust Encoding of Data
            In addition to all the  reasons  given  above  for  possible
            transformation  of  body data, it will sometimes be the case
            that a gateway can tell that the body data, as  given,  will
            not  robustly  survive  the  next  step  of  transport.  For
            example, mail crossing an ASCII-to-EBCDIC gateway will  lose
            information  if certain characters are used.  In such cases,
            a gateway can make the data more robust simply  by  applying
            one of the MIME Content-Transfer-Encoding algorithms (base64
            or quoted-printable) to the body or body  part.   This  will
            generally  be  a  loss-less transformation, but care must be
            taken  to  ensure  that  the  resulting  message  is   MIME-
            conformant  if  the inital message was not.  (For example, a
            MIME-Version header field may need to be added.)
          User-oriented concerns
            If a gateway is going to perform major transformations on  a
            mail  message,  such as translating image formats or mapping
            between included data and external-reference data, it  seems
            inevitable that there will be situations in which users will
            object to these transformations.  This is, in large part, an
            implementation  issue,  but  it  seems  advisable,  wherever
            possible, to provide a mechanism whereby users can  specify,
            to  the  transport  system,  whether  or  not they want such
            services performed automatically on their behalf. The use of
            the  "Content-Conversion"  header field, as mentioned above,
            is suggested for this purpose, since it  it  least  provides
            some control by the sender, if not the recipient.
            [RFC-1341]    Borenstein,   N.,   and   N.   Freed,    "MIME
            (Multipurpose  Internet  Mail  Extensions):  Mechanisms  for
            Specifying and Describing the  Format  of  Internet  Message
            Bodies", RFC 1341, Bellcore, June, 1992.
          Security Considerations
            Security issues are not  discussed in this memo.
          Author's Address
            Nathaniel S. Borenstein
            MRE 2D-296, Bellcore
            445 South St.
            Morristown, NJ 07962-1910
            Phone: +1 201 829 4270
            Fax:  +1 201 829 7019
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