




Network Working Group                                           K. Moore
Request for Comments: 1891                       University of Tennessee
Category: Standards Track                                   January 1996


                         SMTP Service Extension
                   for Delivery Status Notifications

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

1. Abstract

   This memo defines an extension to the SMTP service, which allows an
   SMTP client to specify (a) that delivery status notifications (DSNs)
   should be generated under certain conditions, (b) whether such
   notifications should return the contents of the message, and (c)
   additional information, to be returned with a DSN, that allows the
   sender to identify both the recipient(s) for which the DSN was
   issued, and the transaction in which the original message was sent.

   Any questions, comments, and reports of defects or ambiguities in
   this specification may be sent to the mailing list for the NOTARY
   working group of the IETF, using the address
   <notifications@cs.utk.edu>.  Requests to subscribe to the mailing
   list should be addressed to <notifications-request@cs.utk.edu>.
   Implementors of this specification are encouraged to subscribe to the
   mailing list, so that they will quickly be informed of any problems
   which might hinder interoperability.

   NOTE: This document is a Proposed Standard.  If and when this
   protocol is submitted for Draft Standard status, any normative text
   (phrases containing SHOULD, SHOULD NOT, MUST, MUST NOT, or MAY) in
   this document will be re-evaluated in light of implementation
   experience, and are thus subject to change.

2. Introduction

   The SMTP protocol [1] requires that an SMTP server provide
   notification of delivery failure, if it determines that a message
   cannot be delivered to one or more recipients.  Traditionally, such
   notification consists of an ordinary Internet mail message (format
   defined by [2]), sent to the envelope sender address (the argument of



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   the SMTP MAIL command), containing an explanation of the error and at
   least the headers of the failed message.

   Experience with large mail distribution lists [3] indicates that such
   messages are often insufficient to diagnose problems, or even to
   determine at which host or for which recipients a problem occurred.
   In addition, the lack of a standardized format for delivery
   notifications in Internet mail makes it difficult to exchange such
   notifications with other message handling systems.

   Such experience has demonstrated a need for a delivery status
   notification service for Internet electronic mail, which:

(a) is reliable, in the sense that any DSN request will either be
    honored at the time of final delivery, or result in a response
    that indicates that the request cannot be honored,

(b) when both success and failure notifications are requested,
    provides an unambiguous and nonconflicting indication of whether
    delivery of a message to a recipient succeeded or failed,

(c) is stable, in that a failed attempt to deliver a DSN should never
    result in the transmission of another DSN over the network,

(d) preserves sufficient information to allow the sender to identify
    both the mail transaction and the recipient address which caused
    the notification, even when mail is forwarded or gatewayed to
    foreign environments, and

(e) interfaces acceptably with non-SMTP and non-822-based mail
    systems, both so that notifications returned from foreign mail
    systems may be useful to Internet users, and so that the
    notification requests from foreign environments may be honored.
    Among the requirements implied by this goal are the ability to
    request non-return-of-content, and the ability to specify whether
    positive delivery notifications, negative delivery notifications,
    both, or neither, should be issued.

   In an attempt to provide such a service, this memo uses the mechanism
   defined in [4] to define an extension to the SMTP protocol.  Using
   this mechanism, an SMTP client may request that an SMTP server issue
   or not issue a delivery status notification (DSN) under certain
   conditions.  The format of a DSN is defined in [5].








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3. Framework for the Delivery Status Notification Extension

   The following service extension is therefore defined:

(1) The name of the SMTP service extension is "Delivery Status
    Notification";

(2) the EHLO keyword value associated with this extension is "DSN",
    the meaning of which is defined in section 4 of this memo;

(3) no parameters are allowed with this EHLO keyword value;

(4) two optional parameters are added to the RCPT command, and two
    optional parameters are added to the MAIL command:

    An optional parameter for the RCPT command, using the
    esmtp-keyword "NOTIFY", (to specify the conditions under which a
    delivery status notification should be generated), is defined in
    section 5.1,

    An optional parameter for the RCPT command, using the
    esmtp-keyword "ORCPT", (used to convey the "original"
    (sender-specified) recipient address), is defined in section 5.2,
    and

    An optional parameter for the MAIL command, using the
    esmtp-keyword "RET", (to request that DSNs containing an
    indication of delivery failure either return the entire contents
    of a message or only the message headers), is defined in section
    5.3,

    An optional parameter for the MAIL command, using the
    esmtp-keyword "ENVID", (used to propagate an identifier for this
    message transmission envelope, which is also known to the sender
    and will, if present, be returned in any DSNs issued for this
    transmission), is defined in section 5.4;

(5) no additional SMTP verbs are defined by this extension.

   The remainder of this memo specifies how support for the extension
   effects the behavior of a message transfer agent.

4.  The Delivery Status Notification service extension

   An SMTP client wishing to request a DSN for a message may issue the
   EHLO command to start an SMTP session, to determine if the server
   supports any of several service extensions.  If the server responds
   with code 250 to the EHLO command, and the response includes the EHLO



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   keyword DSN, then the Delivery Status Notification extension (as
   described in this memo) is supported.

   Ordinarily, when an SMTP server returns a positive (2xx) reply code
   in response to a RCPT command, it agrees to accept responsibility for
   either delivering the message to the named recipient, or sending a
   notification to the sender of the message indicating that delivery
   has failed.  However, an extended SMTP ("ESMTP") server which
   implements this service extension will accept an optional NOTIFY
   parameter with the RCPT command. If present, the NOTIFY parameter
   alters the conditions for generation of delivery status notifications
   from the default (issue notifications only on failure) specified in
   [1].  The ESMTP client may also request (via the RET parameter)
   whether the entire contents of the original message should be
   returned (as opposed to just the headers of that message), along with
   the DSN.

   In general, an ESMTP server which implements this service extension
   will propagate delivery status notification requests when relaying
   mail to other SMTP-based MTAs which also support this extension, and
   make a "best effort" to ensure that such requests are honored when
   messages are passed into other environments.

   In order that any delivery status notifications thus generated will
   be meaningful to the sender, any ESMTP server which supports this
   extension will attempt to propagate the following information to any
   other MTAs that are used to relay the message, for use in generating
   DSNs:

(a) for each recipient, a copy of the original recipient address, as
    used by the sender of the message.

    This address need not be the same as the mailbox specified in the
    RCPT command.  For example, if a message was originally addressed
    to A@B.C and later forwarded to A@D.E, after such forwarding has
    taken place, the RCPT command will specify a mailbox of A@D.E.
    However, the original recipient address remains A@B.C.

    Also, if the message originated from an environment which does not
    use Internet-style user@domain addresses, and was gatewayed into
    SMTP, the original recipient address will preserve the original
    form of the recipient address.

(b) for the entire SMTP transaction, an envelope identification
    string, which may be used by the sender to associate any delivery
    status notifications with the transaction used to send the
    original message.




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5.  Additional parameters for RCPT and MAIL commands

   The extended RCPT and MAIL commands are issued by a client when it
   wishes to request a DSN from the server, under certain conditions,
   for a particular recipient.  The extended RCPT and MAIL commands are
   identical to the RCPT and MAIL commands defined in [1], except that
   one or more of the following parameters appear after the sender or
   recipient address, respectively.  The general syntax for extended
   SMTP commands is defined in [4].

   NOTE: Although RFC 822 ABNF is used to describe the syntax of these
   parameters, they are not, in the language of that document,
   "structured field bodies".  Therefore, while parentheses MAY appear
   within an emstp-value, they are not recognized as comment delimiters.

   The syntax for "esmtp-value" in [4] does not allow SP, "=", control
   characters, or characters outside the traditional ASCII range of 1-
   127 decimal to be transmitted in an esmtp-value.  Because the ENVID
   and ORCPT parameters may need to convey values outside this range,
   the esmtp-values for these parameters are encoded as "xtext".
   "xtext" is formally defined as follows:

     xtext = *( xchar / hexchar )

     xchar = any ASCII CHAR between "!" (33) and "~" (126) inclusive,
          except for "+" and "=".

; "hexchar"s are intended to encode octets that cannot appear
; as ASCII characters within an esmtp-value.

     hexchar = ASCII "+" immediately followed by two upper case
          hexadecimal digits

When encoding an octet sequence as xtext:

+ Any ASCII CHAR between "!" and "~" inclusive, except for "+" and "=",
  MAY be encoded as itself.  (A CHAR in this range MAY instead be
  encoded as a "hexchar", at the implementor's discretion.)

+ ASCII CHARs that fall outside the range above must be encoded as
  "hexchar".

5.1  The NOTIFY parameter of the ESMTP RCPT command

   A RCPT command issued by a client may contain the optional esmtp-
   keyword "NOTIFY", to specify the conditions under which the SMTP
   server should generate DSNs for that recipient.  If the NOTIFY
   esmtp-keyword is used, it MUST have an associated esmtp-value,



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   formatted according to the following rules, using the ABNF of RFC
   822:

     notify-esmtp-value = "NEVER" / 1#notify-list-element

     notify-list-element = "SUCCESS" / "FAILURE" / "DELAY"

Notes:

a. Multiple notify-list-elements, separated by commas, MAY appear in a
   NOTIFY parameter; however, the NEVER keyword MUST appear by itself.

b. Any of the keywords NEVER, SUCCESS, FAILURE, or DELAY may be spelled
   in any combination of upper and lower case letters.

The meaning of the NOTIFY parameter values is generally as follows:

+ A NOTIFY parameter value of "NEVER" requests that a DSN not be
  returned to the sender under any conditions.

+ A NOTIFY parameter value containing the "SUCCESS" or "FAILURE"
  keywords requests that a DSN be issued on successful delivery or
  delivery failure, respectively.

+ A NOTIFY parameter value containing the keyword "DELAY" indicates the
  sender's willingness to receive "delayed" DSNs.  Delayed DSNs may be
  issued if delivery of a message has been delayed for an unusual amount
  of time (as determined by the MTA at which the message is delayed),
  but the final delivery status (whether successful or failure) cannot
  be determined.  The absence of the DELAY keyword in a NOTIFY parameter
  requests that a "delayed" DSN NOT be issued under any conditions.

   The actual rules governing interpretation of the NOTIFY parameter are
   given in section 6.

   For compatibility with SMTP clients that do not use the NOTIFY
   facility, the absence of a NOTIFY parameter in a RCPT command may be
   interpreted as either NOTIFY=FAILURE or NOTIFY=FAILURE,DELAY.

5.2 The ORCPT parameter to the ESMTP RCPT command

   The ORCPT esmtp-keyword of the RCPT command is used to specify an
   "original" recipient address that corresponds to the actual recipient
   to which the message is to be delivered.  If the ORCPT esmtp-keyword
   is used, it MUST have an associated esmtp-value, which consists of
   the original recipient address, encoded according to the rules below.
   The ABNF for the ORCPT parameter is:




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     orcpt-parameter = "ORCPT=" original-recipient-address

     original-recipient-address = addr-type ";" xtext

     addr-type = atom

   The "addr-type" portion MUST be an IANA-registered electronic mail
   address-type (as defined in [5]), while the "xtext" portion contains
   an encoded representation of the original recipient address using the
   rules in section 5 of this document.  The entire ORCPT parameter MAY
   be up to 500 characters in length.

   When initially submitting a message via SMTP, if the ORCPT parameter
   is used, it MUST contain the same address as the RCPT TO address
   (unlike the RCPT TO address, the ORCPT parameter will be encoded as
   xtext).  Likewise, when a mailing list submits a message via SMTP to
   be distributed to the list subscribers, if ORCPT is used, the ORCPT
   parameter MUST match the new RCPT TO address of each recipient, not
   the address specified by the original sender of the message.)

   The "addr-type" portion of the original-recipient-address is used to
   indicate the "type" of the address which appears in the ORCPT
   parameter value.  However, the address associated with the ORCPT
   keyword is NOT constrained to conform to the syntax rules for that
   "addr-type".

   Ideally, the "xtext" portion of the original-recipient-address should
   contain, in encoded form, the same sequence of characters that the
   sender used to specify the recipient.  However, for a message
   gatewayed from an environment (such as X.400) in which a recipient
   address is not a simple string of printable characters, the
   representation of recipient address must be defined by a
   specification for gatewaying between DSNs and that environment.

5.3 The RET parameter of the ESMTP MAIL command

   The RET esmtp-keyword on the extended MAIL command specifies whether
   or not the message should be included in any failed DSN issued for
   this message transmission.  If the RET esmtp-keyword is used, it MUST
   have an associated esmtp-value, which is one of the following
   keywords:

   FULL  requests that the entire message be returned in any "failed"
         delivery status notification issued for this recipient.

   HDRS  requests that only the headers of the message be returned.





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   The FULL and HDRS keywords may be spelled in any combination of upper
   and lower case letters.

   If no RET parameter is supplied, the MTA MAY return either the
   headers of the message or the entire message for any DSN containing
   indication of failed deliveries.

   Note that the RET parameter only applies to DSNs that indicate
   delivery failure for at least one recipient.  If a DSN contains no
   indications of delivery failure, only the headers of the message
   should be returned.

5.4  The ENVID parameter to the ESMTP MAIL command

   The ENVID esmtp-keyword of the SMTP MAIL command is used to specify
   an "envelope identifier" to be transmitted along with the message and
   included in any DSNs issued for any of the recipients named in this
   SMTP transaction.  The purpose of the envelope identifier is to allow
   the sender of a message to identify the transaction for which the DSN
   was issued.

   The ABNF for the ENVID parameter is:

     envid-parameter = "ENVID=" xtext

   The ENVID esmtp-keyword MUST have an associated esmtp-value.  No
   meaning is assigned by the mail system to the presence or absence of
   this parameter or to any esmtp-value associated with this parameter;
   the information is used only by the sender or his user agent.  The
   ENVID parameter MAY be up to 100 characters in length.

5.5 Restrictions on the use of Delivery Status Notification parameters

   The RET and ENVID parameters MUST NOT appear more than once each in
   any single MAIL command.  If more than one of either of these
   parameters appears in a MAIL command, the ESMTP server SHOULD respond
   with "501 syntax error in parameters or arguments".

   The NOTIFY and ORCPT parameters MUST NOT appear more than once in any
   RCPT command.  If more than one of either of these parameters appears
   in a RCPT command, the ESMTP server SHOULD respond with "501 syntax
   error in parameters or arguments".

6. Conformance requirements

   The Simple Mail Transfer Protocol (SMTP) is used by Message Transfer
   Agents (MTAs) when accepting, relaying, or gatewaying mail, as well
   as User Agents (UAs) when submitting mail to the mail transport



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   system.  The DSN extension to SMTP may be used to allow UAs to convey
   the sender's requests as to when DSNs should be issued.  A UA which
   claims to conform to this specification must meet certain
   requirements as described below.

   Typically, a message transfer agent (MTA) which supports SMTP will
   assume, at different times, both the role of a SMTP client and an
   SMTP server, and may also provide local delivery, gatewaying to
   foreign environments, forwarding, and mailing list expansion.  An MTA
   which, when acting as an SMTP server, issues the DSN keyword in
   response to the EHLO command, MUST obey the rules below for a
   "conforming SMTP client" when acting as a client, and a "conforming
   SMTP server" when acting as a server.  The term "conforming MTA"
   refers to an MTA which conforms to this specification, independent of
   its role of client or server.

6.1 SMTP protocol interactions

   The following rules apply to SMTP transactions in which any of the
   ENVID, NOTIFY, RET, or ORCPT keywords are used:

(a) If an SMTP client issues a MAIL command containing a valid ENVID
    parameter and associated esmtp-value and/or a valid RET parameter
    and associated esmtp-value, a conforming SMTP server MUST return
    the same reply-code as it would to the same MAIL command without
    the ENVID and/or RET parameters.  A conforming SMTP server MUST
    NOT refuse a MAIL command based on the absence or presence of
    valid ENVID or RET parameters, or on their associated
    esmtp-values.

    However, if the associated esmtp-value is not valid (i.e. contains
    illegal characters), or if there is more than one ENVID or RET
    parameter in a particular MAIL command, the server MUST issue the
    reply-code 501 with an appropriate message (e.g.  "syntax error in
    parameter").

(b) If an SMTP client issues a RCPT command containing any valid
    NOTIFY and/or ORCPT parameters, a conforming SMTP server MUST
    return the same response as it would to the same RCPT command
    without those NOTIFY and/or ORCPT parameters.  A conforming SMTP
    server MUST NOT refuse a RCPT command based on the presence or
    absence of any of these parameters.

    However, if any of the associated esmtp-values are not valid, or
    if there is more than one of any of these parameters in a
    particular RCPT command, the server SHOULD issue the response "501
    syntax error in parameter".




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6.2 Handling of messages received via SMTP

   This section describes how a conforming MTA should handle any
   messages received via SMTP.

   NOTE: A DSN MUST NOT be returned to the sender for any message for
   which the return address from the SMTP MAIL command was NULL ("<>"),
   even if the sender's address is available from other sources (e.g.
   the message header).  However, the MTA which would otherwise issue a
   DSN SHOULD inform the local postmaster of delivery failures through
   some appropriate mechanism that will not itself

