encapsulation boundary.

   Encapsulation boundaries must not appear within the encapsulations,
   and must be no longer than 70 characters, not counting the two
   leading hyphens.

   The encapsulation boundary following the last body part is a
   distinguished delimiter that indicates that no further body parts
   will follow.  Such a delimiter is identical to the previous
   delimiters, with the addition of two more hyphens at the end of the
   line:

                 --gc0p4Jq0M2Yt08jU534c0p--

   There appears to be room for additional information prior to the
   first encapsulation boundary and following the final boundary.  These
   areas should generally be left blank, and implementations must ignore
   anything that appears before the first boundary or after the last
   one.

      NOTE: These "preamble" and "epilogue" areas are generally not used
      because of the lack of proper typing of these parts and the lack
      of clear semantics for handling these areas at gateways,
      particularly X.400 gateways.  However, rather than leaving the
      preamble area blank, many MIME implementations have found this to
      be a convenient place to insert an explanatory note for recipients
      who read the message with pre-MIME software, since such notes will
      be ignored by MIME-compliant software.

      NOTE: Because encapsulation boundaries must not appear in the body
      parts being encapsulated, a user agent must exercise care to
      choose a unique boundary.  The boundary in the example above could
      have been the result of an algorithm designed to produce
      boundaries with a very low probability of already existing in the




 
RFC 1521                          MIME                    September 1993


      data to be encapsulated without having to prescan the data.
      Alternate algorithms might result in more 'readable' boundaries
      for a recipient with an old user agent, but would require more
      attention to the possibility that the boundary might appear in the
      encapsulated part.  The simplest boundary possible is something
      like "---", with a closing boundary of "-----".

   As a very simple example, the following multipart message has two
   parts, both of them plain text, one of them explicitly typed and one
   of them implicitly typed:

      From: Nathaniel Borenstein <nsb@bellcore.com>
      To:  Ned Freed <ned@innosoft.com>
      Subject: Sample message
      MIME-Version: 1.0
      Content-type: multipart/mixed; boundary="simple
      boundary"

      This is the preamble.  It is to be ignored, though it
      is a handy place for mail composers to include an
      explanatory note to non-MIME conformant readers.
      --simple boundary

      This is implicitly typed plain ASCII text.
      It does NOT end with a linebreak.
      --simple boundary
      Content-type: text/plain; charset=us-ascii

      This is explicitly typed plain ASCII text.
      It DOES end with a linebreak.

      --simple boundary--
      This is the epilogue.  It is also to be ignored.

   The use of a Content-Type of multipart in a body part within another
   multipart entity is explicitly allowed.  In such cases, for obvious
   reasons, care must be taken to ensure that each nested multipart
   entity must use a different boundary delimiter. See Appendix C for an
   example of nested multipart entities.

   The use of the multipart Content-Type with only a single body part
   may be useful in certain contexts, and is explicitly permitted.

   The only mandatory parameter for the multipart Content-Type is the
   boundary parameter, which consists of 1 to 70 characters from a set
   of characters known to be very robust through email gateways, and NOT
   ending with white space.  (If a boundary appears to end with white
   space, the white space must be presumed to have been added by a




 
RFC 1521                          MIME                    September 1993


   gateway, and must be deleted.)  It is formally specified by the
   following BNF: