RFC 1428 (rfc1428) - Page 3 of 6

Transition of Internet Mail from Just-Send-8 to 8bit-SMTP/MIME

RFC 1428              Transition to 8bit-SMTP/MIME         February 1993


      characters are in the US-ASCII subset of ISO 8859 may be somewhat
      readable.

   (3) 8bit transparent sender to 8bit transparent receiver

      Will work if an external agreement "out of band" to use a
      particular character set without tagging exists between the sender
      and the receiver.

   (4) 8bit transparent sender to MIME/ESMTP conformant receiver

      Will work if a reasonable upgrade path is provided via gateways,
      the indicated character set tag inserted by the gateway is correct
      and the receiver supports the character set chosen by the sender.
      This case is the focus of this memo.

   (5) MIME/ESMTP sender to non-MIME 7bit receiver

      Because the ESMTP/MIME sender cannot know if the receiver will
      understand 8bits, the sender will encode the text into base-64 or
      quoted-printable which may be considered "garbled" by the
      receiver.  To provide a useful downgrade path the gateway must
      have some knowledge about the capabilities of the receiver. When
      the character set can be clearly identified, techniques like the
      menmonic MNEM encoding described in RFC 1345 may be helpful in
      this case.

   (6) MIME/ESMTP sender to MIME/ESMTP receiver

      Interoperability will be attained provided the receiver supports
      the character set chosen by the sender.

3. Upgrade Path from 8bit Transparent to ESMTP/MIME

   A gateway which has been upgraded to support Extended SMTP may
   upgrade an 8bit message received to MIME.  This is consistent with
   the requirement that all 8bit mail sent by ESMTP be encoded in MIME.
   The upgrade should be done using the best available information.

   A site may "upgrade" to MIME en-masse by implementing MIME conversion
   for all messages leaving the site.  For text messages, the body can
   be converted by adding a MIME-version header and a Content-Type:
   Text/Plain with the character set in use in the site, provided the
   site uses a single character set.

   An appropriate Content-Transfer-Encoding header line must be added to
   indicate any encoding that may be necessary.




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