Implications of MIME for Internet Mail Gateways (RFC1344)

            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.

          Abstract

            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

            Borenstein                                          [Page 1]

            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...