RFC 2442

Network Working Group                                         N. Freed
Request for Comments: 2442                                   D. Newman
Category: Informational                                       Innosoft
                                                          J. Belissent
                                                      Sun Microsystems
                                                                M. Hoy
                                                         November 1998

                               Batch SMTP
                               Media Type

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.


   This document defines a MIME content type suitable for tunneling an
   ESMTP [RFC-821, RFC-1869] transaction through any MIME-capable
   transport.  This type can be used for a variety of purposes,
   including:  Extending end-to-end MIME-based security services (e.g.,
   [RFC-1847]) to cover message envelope information as well as message
   content.  Making it possible to use specific SMTP extensions such as
   NOTARY [RFC-1891] over unextended SMTP transport infrastructure.
   Enabling the transfer of multiple separate messages in a single
   transactional unit.

Requirements Notation

   This document occasionally uses terms that appear in capital letters.
   When the terms "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   appear capitalized, they are being used to indicate particular
   requirements of this specification. A discussion of the meanings of
   the terms "MUST", "SHOULD", and "MAY" appears in [RFC-1123]; the
   terms "MUST NOT" and "SHOULD NOT" are logical extensions of this

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RFC 2442                 Batch SMTP Media Type             November 1998

The Application/batch-SMTP Content Type

   The "application/batch-SMTP" MIME content type is a container for the
   client side of an SMTP or ESMTP transaction. In keeping with
   traditional SMTP, the contents are line oriented and CRLF line
   terminators MUST be used.

   The "application/batch-SMTP" type is defined as follows:

      Media type name: application
      Media subtype name: batch-SMTP
      Required parameters: none
      Optional parameters: required-extensions
      Encoding considerations:
        8bit material may appear, so quoted-printable or base64
        encoding may be necessary on transports that do not
        support 8bit. While the content of this type is
        line-oriented and uses conventional CR/LF terminators,
        lines longer than 7bit and 8bit encodings allow (998
        octets) may appear, hence quoted-printable or
        base64 encoding may be necessary even in conjunction
        with 8bit transports.
      Security considerations:
        Discussed in the Security Considerations Section.

How application/batch-SMTP is used

   The following diagram illustrates how the application/batch-SMTP type
   is intended to be used:

                    application/batch-SMTP object
                         |                |
           +-----------+ v  +----------+  v +-----------+
           | batch     |    | MIME-    |    | batch     |
        => | SMTP      | => | capable  | => | SMTP      | =>
           | generator |    |transport |    | processor |
        ^  +-----------+    +----------+    +-----------+  ^
        |                                                  |
        +-- conventional SMTP/RFC822 message transaction --+

   A conventional SMTP message transaction is converted into an
   application/batch-SMTP object by the batch SMTP generator. This
   object is then carried over some type of MIME-capable transport. Once
   the destination is reached the object is presented to a batch SMTP
   processor, which converts the application/batch-SMTP object back into
   a conventional SMTP message transaction.

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RFC 2442                 Batch SMTP Media Type             November 1998

Generation of application/batch-SMTP material

   Application/batch-SMTP material is generated by a specially modified
   SMTP client operating without a corresponding SMTP server. The client
   simply assumes a successful response to all commands it issues. The
   resulting content then consists of the collected output from the SMTP

Honoring SMTP restrictions

   Most batch SMTP processors will be constructed by modifying and
   extending existing SMTP servers. As such, all of the restrictions on
   SMTP constructs imposed by RFC 821, RFC 1123, and RFC 1869 MUST be
   observed. In particular, restrictions on command and data line
   lengths, number of recipients, and so on still exist and apply to
   batch SMTP.

Use of SMTP Extensions

   Since no SMTP server is present the client must be prepared to make
   certain assumptions about which SMTP extensions can be used. The
   generator MAY assume that ESMTP [RFC-1869] facilities are available,
   that is, it is acceptable to use the EHLO command and additional
   parameters on MAIL FROM and RCPT TO.  If EHLO is used MAY assume that
   the 8bitMIME [RFC-1652], SIZE [RFC-1870], and NOTARY [RFC-1891]
   extensions are available. In particular, NOTARY SHOULD be used.  MAY
   create private bilateral agreements which specify the availability of
   additional SMTP extensions. Additional SMTP extensions MUST NOT be
   used in the absence of such an agreement, and, perhaps more
   importantly, a conformant generation of application/batch-SMTP
   objects MUST be able to produce objects restricted to use of the
   extensions listed above.

   The "required-extensions" content type parameter MAY be used to
   communicate a list of the extensions actually used, specified as a
   comma-separated list of EHLO responses. If absent it defaults to the
   list "8bitMIME,SIZE,NOTARY".  Any use by private bilateral agreement
   of additional or different extensions MUST be noted in the
   "required-extensions" parameter.

   Note that many SMTP extensions simply do not make sense in the
   context of batch SMTP. For example, the pipelining extension [RFC-
   2197] makes no sense in the absence of a network connection.

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RFC 2442                 Batch SMTP Media Type             November 1998

Handling Multiple Messages

   Generators SHOULD attempt to minimize the number of messages placed
   in a single application/batch-SMTP object. Ideally a single
   application/batch-SMTP object will be created for each message. Note,
   however, that some uses of application/batch-SMTP (e.g., mail
   bagging) may exist solely to take advantage of the multiple messages
   in a single container capability of batch SMTP, so requiring one
   message per container is not possible.

   DISCUSSION: The SMTP protocol provides for the transfer of a series
   of messages over a single connection. This extends in a natural way
   to batch SMTP.  However, the issues in batch SMTP are somewhat
   different. Suppose, for example, that a batch SMTP processor receives
   an application/batch-SMTP object containing two messages but is
   unable to process the second message because of a storage allocation
   failure. But suppose that not only does this failure preclude
   processing of the second message, it also precludes recording that
   the first message has already been processed. Subsequent reprocessing
   of the application/batch-SMTP could then lead to duplication of the
   first message.

   This issue is not materially different from the well-known problems
   with SMTP synchronization that in practice often lead to duplicated
   messages. Since this behavior is inherent in SMTP to begin with it is
   not incumbent on application/batch-SMTP to completely address the
   issue. Nevertheless, it seems prudent for application/batch-SMTP to
   try and not make matters even worse.

Transport of application/batch-SMTP objects

   Application/batch-SMTP objects may be transported by any transport
   capable of preserving their MIME labelling, e.g., HTTP or SMTP.

   Transports MUST remain cognizant of the special nature of
   application/batch-SMTP. An application/batch-SMTP object contains one
   or more "frozen" SMTP message transactions. SMTP message transactions
   typically carry with them various assumptions about quality of
   service, e.g., that messages will either be delivered successfully or
   a nondelivery notification will be returned, that a nondelivery
   notification will be returned if delivery cannot be accomplished in a
   timely fashion, and so on. It is vital that the encapsulation of
   these objects for carriage over other forms of transport not
   interfere with these capabilities.

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RFC 2442                 Batch SMTP Media Type             November 1998

Processing of application/batch-SMTP material

   Processing of application/batch-SMTP material is considerably more
   complex than generating it. As might be expected, a modified
   SMTP/ESMTP processor is used.  However, since it cannot return
   information to the client, it must handle all error conditions that
   arise itself. In other words, a batch SMTP processor assumes both the
   responsibilities of a traditional SMTP server as well as part of the
   responsibilities of a traditional SMTP client.

   As such, a conforming processor:  MUST check MIME content type
   information to insure that the material it has been presented with is
   labelled as application/batch-SMTP and doesn't specify any extensions
   the processor doesn't support in the "required-extensions" parameter.
   Application/batch-SMTP objects that employ an unsupported extension
   SHOULD be forwarded to the local postmaster for manual inspection and
   handling.  MUST accept any syntactically valid EHLO or HELO command.
   MUST accept any syntactically valid MAIL FROM command. A conforming
   processor, MAY, if it so desires, note the unacceptability of some
   part of a given MAIL FROM command and use this information to
   subsequently generate non-delivery notifications for any or all
   recipients.  MUST accept any syntactically valid RCPT TO command. A
   conforming processor SHOULD note the unacceptability of some part of
   a given RCPT TO command and subsequently use this information to
   generate a non-delivery notification for this recipient in lieu of
   actually delivering the message.  MUST accept any of the additional
   parameters defined by the 8bitMIME, SIZE, and NOTARY SMTP extensions
   on the MAIL FROM and RCPT TO commands.  MUST accept the DATA command
   even when no valid recipients are present. 8bit MIME messages MUST be
   accepted.  MUST accept the RSET command and handle multiple messages
   in a single application/batch-SMTP object. Processors MUST process
   each message in an application/batch-SMTP object once and SHOULD take
   whatever steps are necessary to avoid processing a message more than
   once. For example, if processing of an application/batch-SMTP object
   containing multiple messages is interrupted at an intermediate point
   it should subsequently be restarted at the end of the last message
   that was completely processed.  SHOULD forward any syntactically
   invalid application/batch-SMTP message to the local postmaster for
   manual inspection and handling.

Security Considerations

   Application/batch-SMTP implements a tunneling mechanism. In general
   tunneling mechanisms are prone to abuse because they may provide a
   means of bypassing existing security restrictions. For example, an
   application/batch-SMTP tunnel implemented over an existing SMTP
   transport may allow someone to bypass relay restrictions imposed to
   block redistribution of spam.

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RFC 2442                 Batch SMTP Media Type             November 1998

   Application/batch-SMTP processors SHOULD implement access
   restrictions designed to limit access to the processor to authorized
   generators only. (Note that this facility may be provided
   automatically if application/batch-SMTP is being used to secure
   message envelope information.)


   The general concept of batch SMTP has been around for a long time.
   One particular type of batch SMTP was defined by Alan Crosswell and
   used on BITNET to overcome BITNET's native 8 character limit on user
   and host names. However, this form of batch SMTP differed from the
   current proposal in that it envisioned having the server return the
   status code responses to the client. In this case the client bore the
   burden of correlating responses with the original SMTP dialogue after
   the fact.

   Unfortunately this approach proved not to work well in practice.
   BITNET eventually switched to the same basic form of batch SMTP that
   has been defined here. Unfortunately that definition was, to the best
   of the present authors' knowledge, never captured in a formal
   specification. It should also be noted that the definition given here
   also differs in that it takes SMTP extensions into account.

   Einar Stefferud had previously considered the problem of carrying
   extended SMTP messages over unextended SMTP transports. He proposed
   that some form of "double enveloping" technology be developed to
   address this problem. The mechanism presented here effectively
   implements the type of solution he proposed.


   [RFC-821]  Postel, J., "Simple Mail Transfer Protocol", STD 10,
              RFC 821, August 1982.

   [RFC-822]  Crocker, D., "Standard for the Format of ARPA Internet
              Text Messages", STD 11, RFC 822 August 1982.

   [RFC-1123] Braden, B., "Requirements for Internet Hosts --
              Application and Support", STD 3, RFC 1123, October 1989.

   [RFC-1652] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.
              Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
              RFC 1652, July 1994.

   [RFC-1847] Galvin, J., Murphy, S., Crocker, S. and N. Freed,
              "Security Multiparts for MIME:  Multipart/Signed and
              Multipart/Encrypted", RFC 1847, October 1995.

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RFC 2442                 Batch SMTP Media Type             November 1998

   [RFC-1869] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.
              Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
              November 1995.

   [RFC-1870] Klensin, J., Freed, N., Moore, K., "SMTP Service Extension
              for Message Size Declaration", STD 10, RFC 1870, November,

   [RFC-2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, December 1996.

   [RFC-2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              December 1996.

   [RFC-2197] Freed, N. and A. Cargille, "SMTP Service Extension for
              Command Pipelining", RFC 2197, September 1997.

Authors' Addresses

   Ned Freed
   Innosoft International, Inc.
   1050 Lakes Drive
   West Covina, CA 91790

   Phone: +1 626 919 3600
   Fax:   +1 626 919 3614
   EMail: ned.freed@innosoft.com

   Dan Newman
   Innosoft International, Inc.
   1050 Lakes Drive
   West Covina, CA 91790

   Phone: +1 626 919 3600
   Fax:   +1 626 919 3614
   EMail: dan.newman@innosoft.com

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RFC 2442                 Batch SMTP Media Type             November 1998

   Mark Hoy
   Mainbrace Corporation
   1136 West Evelyn Avenue
   Sunnyvale, CA 94086

   tel: +1 408 774 5265
   fax: +1 408 774 5290
   email: mark.hoy@mainbrace.com

   Jacques Bellisent

   Phone: +1 650 786 3630
   EMail: jacques.belissent@eng.sun.com

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RFC 2442                 Batch SMTP Media Type             November 1998

Full Copyright Statement

   Copyright (C) The Internet Society (1998). All Rights Reserved.

   This document and translations of it may be copied and furnished  to
   others, and derivative works that comment on or otherwise  explain it
   or assist in its implementation may be prepared, copied,  published
   and distributed, in whole or in part, without  restriction of any
   kind, provided that the above copyright notice  and this paragraph
   are included on all such copies and derivative  works.  However, this
   document itself may not be modified in any  way, such as by removing
   the copyright notice or references to the  Internet Society or other
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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on  an

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