RFC 1344 (rfc1344) - Page 2 of 9

Implications of MIME for Internet Mail Gateways

            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
            gateway.

            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
            large).

            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



            Borenstein