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


                         <span class="h1">SMTP Service Extension</span>
                   <span class="h1">for Delivery Status Notifications</span>

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.

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Abstract</span>

   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
   &lt;notifications@cs.utk.edu&gt;.  Requests to subscribe to the mailing
   list should be addressed to &lt;notifications-request@cs.utk.edu&gt;.
   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.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Introduction</span>

   The SMTP protocol [<a href="#ref-1" title="&quot;Simple Mail Transfer Protocol&quot;">1</a>] 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 [<a href="#ref-2" title="&quot;Standard for the Format of ARPA Internet Text Messages&quot;">2</a>]), 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 [<a href="#ref-3" title="&quot;Problems with the Maintenance of Large Mailing Lists.&quot;">3</a>] 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 [<a href="#ref-4" title="&quot;SMTP Service Extensions&quot;">4</a>] 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 [<a href="#ref-5" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">5</a>].








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<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Framework for the Delivery Status Notification Extension</span>

   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 <a href="#section-4">section 4</a> 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
    <a href="#section-5.1">section 5.1</a>,

    An optional parameter for the RCPT command, using the
    esmtp-keyword "ORCPT", (used to convey the "original"
    (sender-specified) recipient address), is defined in <a href="#section-5.2">section 5.2</a>,
    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 <a href="#section-5.3">section</a>
    <a href="#section-5.3">5.3</a>,

    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 <a href="#section-5.4">section 5.4</a>;

(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.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  The Delivery Status Notification service extension</span>

   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
   [<a href="#ref-1" title="&quot;Simple Mail Transfer Protocol&quot;">1</a>].  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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<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Additional parameters for RCPT and MAIL commands</span>

   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 [<a href="#ref-1" title="&quot;Simple Mail Transfer Protocol&quot;">1</a>], 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 [<a href="#ref-4" title="&quot;SMTP Service Extensions&quot;">4</a>].

   NOTE: Although <a href="./rfc822">RFC 822</a> 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 [<a href="#ref-4" title="&quot;SMTP Service Extensions&quot;">4</a>] 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".

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>  The NOTIFY parameter of the ESMTP RCPT command</span>

   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 <a href="./rfc822">RFC</a>
   <a href="./rfc822">822</a>:

     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 <a href="#section-6">section 6</a>.

   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.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a> The ORCPT parameter to the ESMTP RCPT command</span>

   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 [<a href="#ref-5" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">5</a>]), while the "xtext" portion contains
   an encoded representation of the original recipient address using the
   rules in <a href="#section-5">section 5</a> 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.

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a> The RET parameter of the ESMTP MAIL command</span>

   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.

<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>  The ENVID parameter to the ESMTP MAIL command</span>

   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.

<span class="h3"><a class="selflink" id="section-5.5" href="#section-5.5">5.5</a> Restrictions on the use of Delivery Status Notification parameters</span>

   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".

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Conformance requirements</span>

   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.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a> SMTP protocol interactions</span>

   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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<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a> Handling of messages received via SMTP</span>

   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 ("&lt;&gt;"),
   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 result in the
   generation of DSNs.

   DISCUSSION: <a href="./rfc1123#section-2.3.3">RFC 1123, section&nbsp;2.3.3</a> requires error notifications to
   be sent with a NULL return address ("reverse-path").  This creates an
   interesting situation when a message arrives with one or more
   nonfunctional recipient addresses in addition to a nonfunctional
   return address.  When delivery to one of the recipient addresses
   fails, the MTA will attempt to send a nondelivery notification to the
   return address, setting the return address on the notification to
   NULL.  When the delivery of this notification fails, the MTA
   attempting delivery of that notification sees a NULL return address.
   If that MTA were not to inform anyone of the situation, the original
   message would be silently lost.  Furthermore, a nonfunctional return
   address is often indicative of a configuration problem in the
   sender's MTA.  Reporting the condition to the local postmaster may
   help to speed correction of such errors.

<span class="h4"><a class="selflink" id="section-6.2.1" href="#section-6.2.1">6.2.1</a> Relay of messages to other conforming SMTP servers</span>

   The following rules govern the behavior of a conforming MTA, when
   relaying a message which was received via the SMTP protocol, to an
   SMTP server that supports the Delivery Status Notification service
   extension:

(a) Any ENVID parameter included in the MAIL command when a message was
    received, MUST also appear on the MAIL command with which the
    message is relayed, with the same associated esmtp-value.  If no
    ENVID parameter was included in the MAIL command when the message
    was received, the ENVID parameter MUST NOT be supplied when the
    message is relayed.

(b) Any RET parameter included in the MAIL command when a message was
    received, MUST also appear on the MAIL command with which the
    message is relayed, with the same associated esmtp-value.  If no RET
    parameter was included in the MAIL command when the message was
    received, the RET parameter MUST NOT supplied when the message is
    relayed.



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(c) If the NOTIFY parameter was supplied for a recipient when the
    message was received, the RCPT command issued when the message is
    relayed MUST also contain the NOTIFY parameter along with its
    associated esmtp-value.  If the NOTIFY parameter was not supplied
    for a recipient when the message was received, the NOTIFY parameter
    MUST NOT be supplied for that recipient when the message is relayed.

(d) If any ORCPT parameter was present in the RCPT command for a
    recipient when the message was received, an ORCPT parameter with the
    identical original-recipient-address MUST appear in the RCPT command
    issued for that recipient when relaying the message.  (For example,
    the MTA therefore MUST NOT change the case of any alphabetic
    characters in an ORCPT parameter.)

    If no ORCPT parameter was present in the RCPT command when the
    message was received, an ORCPT parameter MAY be added to the RCPT
    command when the message is relayed.  If an ORCPT parameter is added
    by the relaying MTA, it MUST contain the recipient address from the
    RCPT command used when the message was received by that MTA.

<span class="h4"><a class="selflink" id="section-6.2.2" href="#section-6.2.2">6.2.2</a>  Relay of messages to non-conforming SMTP servers</span>

   The following rules govern the behavior of a conforming MTA (in the
   role of client), when relaying a message which was received via the
   SMTP protocol, to an SMTP server that does not support the Delivery
   Status Notification service extension:

(a) ENVID, NOTIFY, RET, or ORCPT parameters MUST NOT be issued when
    relaying the message.

(b) If the NOTIFY parameter was supplied for a recipient, with an esmtp-
    value containing the keyword SUCCESS, and the SMTP server returns a
    success (2xx) reply-code in response to the RCPT command, the client
    MUST issue a "relayed" DSN for that recipient.

(c) If the NOTIFY parameter was supplied for a recipient with an esmtp-
    value containing the keyword FAILURE, and the SMTP server returns a
    permanent failure (5xx) reply-code in response to the RCPT command,
    the client MUST issue a "failed" DSN for that recipient.

(d) If the NOTIFY parameter was supplied for a recipient with an esmtp-
    value of NEVER, the client MUST NOT issue a DSN for that recipient,
    regardless of the reply-code returned by the SMTP server.  However,
    if the server returned a failure (5xx) reply-code, the client MAY
    inform the local postmaster of the delivery failure via an
    appropriate mechanism that will not itself result in the generation
    of DSNs.




<span class="grey">Moore                       Standards Track                    [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


    When attempting to relay a message to an SMTP server that does not
    support this extension, and if NOTIFY=NEVER was specified for some
    recipients of that message, a conforming SMTP client MAY relay the
    message for those recipients in a separate SMTP transaction, using
    an empty reverse-path in the MAIL command.  This will prevent DSNs
    from being issued for those recipients by MTAs that conform to [<a href="#ref-1" title="&quot;Simple Mail Transfer Protocol&quot;">1</a>].

(e) If a NOTIFY parameter was not supplied for a recipient, and the SMTP
    server returns a success (2xx) reply-code in response to a RCPT
    command, the client MUST NOT issue any DSN for that recipient.

(f) If a NOTIFY parameter was not supplied for a recipient, and the SMTP
    server returns a permanent failure (5xx) reply-code in response to a
    RCPT command, the client MUST issue a "failed" DSN for that
    recipient.

<span class="h4"><a class="selflink" id="section-6.2.3" href="#section-6.2.3">6.2.3</a>  Local delivery of messages</span>

   The following rules govern the behavior of a conforming MTA upon
   successful delivery of a message that was received via the SMTP
   protocol, to a local recipient's mailbox:

   "Delivery" means that the message has been placed in the recipient's
   mailbox.  For messages which are transmitted to a mailbox for later
   retrieval via IMAP [<a href="#ref-6" title="&quot;Internet Message Access Protocol - Version 4&quot;">6</a>], POP [<a href="#ref-7" title="&quot;Post Office Protocol - Version 3&quot;">7</a>] or a similar message access protocol,
   "delivery" occurs when the message is made available to the IMAP
   (POP, etc.) service, rather than when the message is retrieved by the
   recipient's user agent.

   Similarly, for a recipient address which corresponds to a mailing
   list exploder, "delivery" occurs when the message is made available
   to that list exploder, even though the list exploder might refuse to
   deliver that message to the list recipients.

(a) If the NOTIFY parameter was supplied for that recipient, with an
    esmtp-value containing the SUCCESS keyword, the MTA MUST issue a
    "delivered" DSN for that recipient.

(b) If the NOTIFY parameter was supplied for that recipient which did
    not contain the SUCCESS keyword, the MTA MUST NOT issue a DSN for
    that recipient.

(c) If the NOTIFY parameter was not supplied for that recipient, the MTA
    MUST NOT issue a DSN.







<span class="grey">Moore                       Standards Track                    [Page 12]</span></pre>
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<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


<span class="h4"><a class="selflink" id="section-6.2.4" href="#section-6.2.4">6.2.4</a>  Gatewaying a message into a foreign environment</span>

   The following rules govern the behavior of a conforming MTA, when
   gatewaying a message that was received via the SMTP protocol, into a
   foreign (non-SMTP) environment:

(a) If the the foreign environment is capable of issuing appropriate
    notifications under the conditions requested by the NOTIFY
    parameter, and the conforming MTA can ensure that any notification
    thus issued will be translated into a DSN and delivered to the
    original sender, then the MTA SHOULD gateway the message into the
    foreign environment, requesting notification under the desired
    conditions, without itself issuing a DSN.

(b) If a NOTIFY parameter was supplied with the SUCCESS keyword, but the
    destination environment cannot return an appropriate notification on
    successful delivery, the MTA SHOULD issue a "relayed" DSN for that
    recipient.

(c) If a NOTIFY parameter was supplied with an esmtp-keyword of NEVER, a
    DSN MUST NOT be issued.  If possible, the MTA SHOULD direct the
    destination environment to not issue delivery notifications for that
    recipient.

(d) If the NOTIFY parameter was not supplied for a particular recipient,
    a DSN SHOULD NOT be issued by the gateway. The gateway SHOULD
    attempt to ensure that appropriate notification will be provided by
    the foreign mail environment if eventual delivery failure occurs,
    and that no notification will be issued on successful delivery.

(e) When gatewaying a message into a foreign environment, the return-of-
    content conditions specified by any RET parameter are nonbinding;
    however, the MTA SHOULD attempt to honor the request using whatever
    mechanisms exist in the foreign environment.

<span class="h4"><a class="selflink" id="section-6.2.5" href="#section-6.2.5">6.2.5</a>  Delays in delivery</span>

   If a conforming MTA receives a message via the SMTP protocol, and is
   unable to deliver or relay the message to one or more recipients for
   an extended length of time (to be determined by the MTA), it MAY
   issue a "delayed" DSN for those recipients, subject to the following
   conditions:

(a) If the NOTIFY parameter was supplied for a recipient and its value
    included the DELAY keyword, a "delayed" DSN MAY be issued.

(b) If the NOTIFY parameter was not supplied for a recipient, a
    "delayed" DSN MAY be issued.



<span class="grey">Moore                       Standards Track                    [Page 13]</span></pre>
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<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


(c) If the NOTIFY parameter was supplied which did not contain the DELAY
    keyword, a "delayed" DSN MUST NOT be issued.

   NOTE: Although delay notifications are common in present-day
   electronic mail, a conforming MTA is never required to issue
   "delayed" DSNs.  The DELAY keyword of the NOTIFY parameter is
   provided to allow the SMTP client to specifically request (by
   omitting the DELAY parameter) that "delayed" DSNs NOT be issued.

<span class="h4"><a class="selflink" id="section-6.2.6" href="#section-6.2.6">6.2.6</a>  Failure of a conforming MTA to deliver a message</span>

   The following rules govern the behavior of a conforming MTA which
   received a message via the SMTP protocol, and is unable to deliver a
   message to a recipient specified in the SMTP transaction:

(a) If a NOTIFY parameter was supplied for the recipient with an esmtp-
    keyword containing the value FAILURE, a "failed" DSN MUST be issued
    by the MTA.

(b) If a NOTIFY parameter was supplied for the recipient which did not
    contain the value FAILURE, a DSN MUST NOT be issued for that
    recipient.  However, the MTA MAY inform the local postmaster of the
    delivery failure via some appropriate mechanism which does not
    itself result in the generation of DSNs.

(c) If no NOTIFY parameter was supplied for the recipient, a "failed"
    DSN MUST be issued.

   NOTE: Some MTAs are known to forward undeliverable messages to the
   local postmaster or "dead letter" mailbox.  This is still considered
   delivery failure, and does not diminish the requirement to issue a
   "failed" DSN under the conditions defined elsewhere in this memo.  If
   a DSN is issued for such a recipient, the Action value MUST be
   "failed".

<span class="h4"><a class="selflink" id="section-6.2.7" href="#section-6.2.7">6.2.7</a> Forwarding, aliases, and mailing lists</span>

   Delivery of a message to a local email address usually causes the
   message to be stored in the recipient's mailbox.  However, MTAs
   commonly provide a facility where a local email address can be
   designated as an "alias" or "mailing list"; delivery to that address
   then causes the message to be forwarded to each of the (local or
   remote) recipient addresses associated with the alias or list.  It is
   also common to allow a user to optionally "forward" her mail to one
   or more alternate addresses.  If this feature is enabled, her mail is
   redistributed to those addresses instead of being deposited in her
   mailbox.




<span class="grey">Moore                       Standards Track                    [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


   Following the example of [<a href="#ref-9" title="&quot;Requirements for Internet Hosts - Application and Support&quot;">9</a>] (<a href="#section-5.3.6">section 5.3.6</a>), this document defines
   the difference between an "alias" and "mailing list" as follows: When
   forwarding a message to the addresses associated with an "alias", the
   envelope return address (e.g. SMTP MAIL FROM) remains intact.
   However, when forwarding a message to the addresses associated with a
   "mailing list", the envelope return address is changed to that of the
   administrator of the mailing list.  This causes DSNs and other
   nondelivery reports resulting from delivery to the list members to be
   sent to the list administrator rather than the sender of the original
   message.

   The DSN processing for aliases and mailing lists is as follows:

<span class="h5"><a class="selflink" id="section-6.2.7.1" href="#section-6.2.7.1">6.2.7.1</a> mailing lists</span>

   When a message is delivered to a list submission address (i.e. placed
   in the list's mailbox for incoming mail, or accepted by the process
   that redistributes the message to the list subscribers), this is
   considered final delivery for the original message.  If the NOTIFY
   parameter for the list submission address contained the SUCCESS
   keyword, a "delivered" DSN MUST be returned to the sender of the
   original message.

   NOTE: Some mailing lists are able to reject message submissions,
   based on the content of the message, the sender's address, or some
   other criteria.  While the interface between such a mailing list and
   its MTA is not well-defined, it is important that DSNs NOT be issued
   by both the MTA (to report successful delivery to the list), and the
   list (to report message rejection using a "failure" DSN.)

   However, even if a "delivered" DSN was issued by the MTA, a mailing
   list which rejects a message submission MAY notify the sender that
   the message was rejected using an ordinary message instead of a DSN.

   Whenever a message is redistributed to an mailing list,

(a) The envelope return address is rewritten to point to the list
    maintainer.  This address MAY be that of a process that recognizes
    DSNs and processes them automatically, but it MUST forward
    unrecognized messages to the human responsible for the list.

(b) The ENVID, NOTIFY, RET, and ORCPT parameters which accompany the
    redistributed message MUST NOT be derived from those of the original
    message.

(c) The NOTIFY and RET parameters MAY be specified by the local
    postmaster or the list administrator.  If ORCPT parameters are
    supplied during redistribution to the list subscribers, they SHOULD



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


    contain the addresses of the list subscribers in the format used by
    the mailing list.

<span class="h5"><a class="selflink" id="section-6.2.7.2" href="#section-6.2.7.2">6.2.7.2</a> single-recipient aliases</span>

   Under normal circumstances, when a message arrives for an "alias"
   which has a single forwarding address, a DSN SHOULD NOT be issued.
   Any ENVID, NOTIFY, RET, or ORCPT parameters SHOULD be propagated with
   the message as it is redistributed to the forwarding address.

<span class="h5"><a class="selflink" id="section-6.2.7.3" href="#section-6.2.7.3">6.2.7.3</a> multiple-recipient aliases</span>

   An "alias" with multiple recipient addresses may be handled in any of
   the following ways:

(a) Any ENVID, NOTIFY, RET, or ORCPT parameters are NOT propagated when
    relaying the message to any of the forwarding addresses.  If the
    NOTIFY parameter for the alias contained the SUCCESS keyword, the
    MTA issues a "relayed" DSN.  (In effect, the MTA treats the message
    as if it were being relayed into an environment that does not
    support DSNs.)

(b) Any ENVID, NOTIFY, RET, or ORCPT parameters (or the equivalent
    requests if the message is gatewayed) are propagated to EXACTLY one
    of the forwarding addresses.  No DSN is issued.  (This is
    appropriate when aliasing is used to forward a message to a
    "vacation" auto-responder program in addition to the local mailbox.)

(c) Any ENVID, RET, or ORCPT parameters are propagated to all forwarding
    addresses associated with that alias.  The NOTIFY parameter is
    propagated to the forwarding addresses, except that it any SUCCESS
    keyword is removed.  If the original NOTIFY parameter for the alias
    contained the SUCCESS keyword, an "expanded" DSN is issued for the
    alias.  If the NOTIFY parameter for the alias did not contain the
    SUCCESS keyword, no DSN is issued for the alias.

<span class="h5"><a class="selflink" id="section-6.2.7.4" href="#section-6.2.7.4">6.2.7.4</a> confidential forwarding addresses</span>

   If it is desired to maintain the confidentiality of a recipient's
   forwarding address, the forwarding may be treated as if it were a
   mailing list.  A DSN will be issued, if appropriate, upon "delivery"
   to the recipient address specified by the sender.  When the message
   is forwarded it will have a new envelope return address. Any DSNs
   which result from delivery failure of the forwarded message will not
   be returned to the original sender of the message and thus not expose
   the recipient's forwarding address.





<span class="grey">Moore                       Standards Track                    [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


<span class="h4"><a class="selflink" id="section-6.2.8" href="#section-6.2.8">6.2.8</a> DSNs describing delivery to multiple recipients</span>

   A single DSN may describe attempts to deliver a message to multiple
   recipients of that message.  If a DSN is issued for some recipients
   in an SMTP transaction and not for others according to the rules
   above, the DSN SHOULD NOT contain information for recipients for whom
   DSNs would not otherwise have been issued.

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a> Handling of messages from other sources</span>

   For messages which originated from "local" users (whatever that
   means), the specifications under which DSNs should be generated can
   be communicated to the MTA via any protocol agreed on between the
   sender's mail composer (user agent) and the MTA.  The local MTA can
   then either relay the message, or issue appropriate delivery status
   notifications.  However, if such requests are transmitted within the
   message itself (for example in the message headers), the requests
   MUST be removed from the message before it is transmitted via SMTP.

   For messages gatewayed from non-SMTP sources and further relayed by
   SMTP, the gateway SHOULD, using the SMTP extensions described here,
   attempt to provide the delivery reporting conditions expected by the
   source mail environment.  If appropriate, any DSNs returned to the
   source environment SHOULD be translated into the format expected in
   that environment.

<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>  Implementation limits</span>

   A conforming MTA MUST accept ESMTP parameters of at least the
   following sizes:

   (a) ENVID parameter: 100 characters.

   (b) NOTIFY parameter: 28 characters.

   (c) ORCPT parameter: 500 characters.

   (d) RET parameter: 8 characters.

   The maximum sizes for the ENVID and ORCPT parameters are intended to
   be adequate for the transmission of "foreign" envelope identifier and
   original recipient addresses.  However, user agents which use SMTP as
   a message submission protocol SHOULD NOT generate ENVID parameters
   which are longer than 38 characters in length.

   A conforming MTA MUST be able to accept SMTP command-lines which are
   at least 1036 characters long (530 characters for the ORCPT and
   NOTIFY parameters of the RCPT command, in addition to the 512



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<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


   characters required by [<a href="#ref-1" title="&quot;Simple Mail Transfer Protocol&quot;">1</a>]).  If other SMTP extensions are supported
   by the MTA, the MTA MUST be able to accept a command-line large
   enough for each SMTP command and any combination of ESMTP parameters
   which may be used with that command.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Format of delivery notifications</span>

   The format of delivery status notifications is defined in [<a href="#ref-5" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">5</a>], which
   uses the framework defined in [<a href="#ref-8" title="&quot;The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages&quot;">8</a>].  Delivery status notifications are
   to be returned to the sender of the original message as outlined
   below.

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a> SMTP Envelope to be used with delivery status notifications</span>

   The DSN sender address (in the SMTP MAIL command) MUST be a null
   reverse-path ("&lt;&gt;"), as required by section 5.3.3 of [<a href="#ref-9" title="&quot;Requirements for Internet Hosts - Application and Support&quot;">9</a>].  The DSN
   recipient address (in the RCPT command) is copied from the MAIL
   command which accompanied the message for which the DSN is being
   issued.  When transmitting a DSN via SMTP, the RET parameter MUST NOT
   be used.  The NOTIFY parameter MAY be used, but its value MUST be
   NEVER.  The ENVID parameter (with a newly generated envelope-id)
   and/or ORCPT parameter MAY be used.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a> Contents of the DSN</span>

   A DSN is transmitted as a MIME message with a top-level content-type
   of multipart/report (as defined in [<a href="#ref-5" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">5</a>]).

   The multipart/report content-type may be used for any of several
   kinds of reports generated by the mail system.  When multipart/report
   is used to convey a DSN, the report-type parameter of the
   multipart/report content-type is "delivery-status".

   As described in [<a href="#ref-8" title="&quot;The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages&quot;">8</a>], the first component of a multipart/report
   content-type is a human readable explanation of the report.  For a
   DSN, the second component of the multipart/report is of content-type
   message/delivery-status (defined in [<a href="#ref-5" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">5</a>]).  The third component of the
   multipart/report consists of the original message or some portion
   thereof.  When the value of the RET parameter is FULL, the full
   message SHOULD be returned for any DSN which conveys notification of
   delivery failure.  (However, if the length of the message is greater
   than some implementation-specified length, the MTA MAY return only
   the headers even if the RET parameter specified FULL.)  If a DSN
   contains no notifications of delivery failure, the MTA SHOULD return
   only the headers.

   The third component must have an appropriate content-type label.
   Issues concerning selection of the content-type are discussed in [<a href="#ref-8" title="&quot;The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages&quot;">8</a>].



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<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a> Message/delivery-status fields</span>

   The message/delivery-status content-type defines a number of fields,
   with general specifications for their contents.  The following
   requirements for any DSNs generated in response to a message received
   by the SMTP protocol by a conforming SMTP server, are in addition to
   the requirements defined in [<a href="#ref-5" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">5</a>] for the message/delivery-status type.

   When generating a DSN for a message which was received via the SMTP
   protocol, a conforming MTA will generate the following fields of the
   message/delivery-status body part:

(a) if an ENVID parameter was present on the MAIL command, an Original-
    Envelope-ID field MUST be supplied, and the value associated with
    the ENVID parameter must appear in that field.  If the message was
    received via SMTP with no ENVID parameter, the Original-Envelope-ID
    field MUST NOT be supplied.

    Since the ENVID parameter is encoded as xtext, but the Original-
    Envelope-ID header is NOT encoded as xtext, the MTA must decode the
    xtext encoding when copying the ENVID value to the Original-
    Envelope-ID field.

(b) The Reporting-MTA field MUST be supplied.  If Reporting MTA can
    determine its fully-qualified Internet domain name, the MTA-name-
    type subfield MUST be "dns", and the field MUST contain the fully-
    qualified domain name of the Reporting MTA. If the fully-qualified
    Internet domain name of the Reporting MTA is not known (for example,
    for an SMTP server which is not directly connected to the Internet),
    the Reporting-MTA field may contain any string identifying the MTA,
    however, in this case the MTA-name-type subfield MUST NOT be "dns".
    A MTA-name-type subfield value of "x-local-hostname" is suggested.

(c) Other per-message fields as defined in [<a href="#ref-5" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">5</a>] MAY be supplied as
    appropriate.

(d) If the ORCPT parameter was provided for this recipient, the
    Original-Recipient field MUST be supplied, with its value taken from
    the ORCPT parameter.  If no ORCPT parameter was provided for this
    recipient, the Original-Recipient field MUST NOT appear.

(e) The Final-Recipient field MUST be supplied. It MUST contain the
    recipient address from the message envelope.  If the message was
    received via SMTP, the address-type will be "<a href="./rfc822">rfc822</a>".

(f) The Action field MUST be supplied.





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<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


(g) The Status field MUST be supplied, using a status-code from [<a href="#ref-10" title="&quot;Enhanced Mail System Status Codes&quot;">10</a>].
    If there is no specific code which suitably describes a delivery
    failure, either 4.0.0 (temporary failure), or 5.0.0 (permanent
    failure) MUST be used.

(h) For DSNs resulting from attempts to relay a message to one or more
    recipients via SMTP, the Remote-MTA field MUST be supplied for each
    of those recipients.  The mta-name-type subfields of those Remote-
    MTA fields will be "dns".

(i) For DSNs resulting from attempts to relay a message to one or more
    recipients via SMTP, the Diagnostic-Code MUST be supplied for each
    of those recipients.  The diagnostic-type subfield will be "smtp".
    See <a href="#section-9.2">section 9.2</a>(a) of this document for a description of the "smtp"
    diagnostic-code.

(j) For DSNs resulting from attempts to relay a message to one or more
    recipients via SMTP, an SMTP-Remote-Recipient extension field MAY be
    supplied for each recipient, which contains the address of that
    recpient which was presented to the remote SMTP server.

(k) Other per-recipient fields defined in [<a href="#ref-5" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">5</a>] MAY appear, as
    appropriate.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Acknowledgments</span>

   The author wishes to thank Eric Allman, Harald Alvestrand, Jim
   Conklin, Bryan Costales, Peter Cowen, Dave Crocker, Roger Fajman, Ned
   Freed, Marko Kaittola, Steve Kille, John Klensin, Anastasios
   Kotsikonas, John Gardiner Myers, Julian Onions, Jacob Palme, Marshall
   Rose, Greg Vaudreuil, and Klaus Weide for their suggestions for
   improvement of this document.



















<span class="grey">Moore                       Standards Track                    [Page 20]</span></pre>
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<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. Appendix - Type-Name Definitions</span>

   The following type names are defined for use in DSN fields generated
   by conforming SMTP-based MTAs:

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a> "<a href="./rfc822">rfc822</a>" address-type</span>

   The "<a href="./rfc822">rfc822</a>" address-type is to be used when reporting Internet
   electronic mail address in the Original-Recipient and Final-Recipient
   DSN fields.

(a) address-type name: <a href="./rfc822">rfc822</a>

(b) syntax for mailbox addresses

    <a href="./rfc822">RFC822</a> mailbox addresses are generally expected to be of the form

    [<a id="ref-route">route</a>] addr-spec

    where "route" and "addr-spec" are defined in [<a href="#ref-2" title="&quot;Standard for the Format of ARPA Internet Text Messages&quot;">2</a>], and the "domain"
    portions of both "route" and "addr-spec" are fully-qualified domain
    names that are registered in the DNS.  However, an MTA MUST NOT
    modify an address obtained from the message envelope to force it to
    conform to syntax rules.

(c) If addresses of this type are not composed entirely of graphic
characters from the US-ASCII repertoire, a specification for how they
are to be encoded as graphic US-ASCII characters in a DSN Original-
Recipient or Final-Recipient DSN field.

    <a href="./rfc822">RFC822</a> addresses consist entirely of graphic characters from the US-
    ASCII repertoire, so no translation is necessary.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a> "smtp" diagnostic-type</span>

   The "smtp" diagnostic-type is to be used when reporting SMTP reply-
   codes in Diagnostic-Code DSN fields.

(a) diagnostic-type name: SMTP

(b) A description of the syntax to be used for expressing diagnostic
codes of this type as graphic characters from the US-ASCII repertoire.

    An SMTP diagnostic-code is of the form

    *( 3*DIGIT "-" *text ) 3*DIGIT SPACE *text





<span class="grey">Moore                       Standards Track                    [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


    For a single-line SMTP reply to an SMTP command, the diagnostic-code
    SHOULD be an exact transcription of the reply.  For multi-line SMTP
    replies, it is necessary to insert a SPACE before each line after
    the first.  For example, an SMTP reply of:

    550-mailbox unavailable
    550 user has moved with no forwarding address

    could appear as follows in a Diagnostic-Code DSN field:

    Diagnostic-Code: smtp ; 550-mailbox unavailable
     550 user has moved with no forwarding address

(c) A list of valid diagnostic codes of this type and the meaning of
each code.

    SMTP reply-codes are currently defined in [<a href="#ref-1" title="&quot;Simple Mail Transfer Protocol&quot;">1</a>], [<a href="#ref-4" title="&quot;SMTP Service Extensions&quot;">4</a>], and [<a href="#ref-9" title="&quot;Requirements for Internet Hosts - Application and Support&quot;">9</a>].
    Additional codes may be defined by other RFCs.

<span class="h3"><a class="selflink" id="section-9.3" href="#section-9.3">9.3</a> "dns" MTA-name-type</span>

   The "dns" MTA-name-type should be used in the Reporting-MTA field.
   An MTA-name of type "dns" is a fully-qualified domain name.  The name
   must be registered in the DNS, and the address Postmaster@{mta-name}
   must be valid.

(a) MTA-name-type name: dns

(b) A description of the syntax of MTA names of this type, using BNF,
regular expressions, ASN.1, or other non-ambiguous language.

    MTA names of type "dns" SHOULD be valid Internet domain names.  If
    such domain names are not available, a domain-literal containing the
    internet protocol address is acceptable.  Such domain names
    generally conform to the following syntax:

    domain = real-domain / domain-literal

    real-domain = sub-domain *("." sub-domain)

    sub-domain = atom

    domain-literal = "[" 1*3DIGIT 3("." 1*3DIGIT) "]"

    where "atom" and "DIGIT" are defined in [<a href="#ref-2" title="&quot;Standard for the Format of ARPA Internet Text Messages&quot;">2</a>].






<span class="grey">Moore                       Standards Track                    [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


(c) If MTA names of this type do not consist entirely of graphic
characters from the US-ASCII repertoire, a specification for how an MTA
name of this type should be expressed as a sequence of graphic US-ASCII
characters.

    MTA names of type "dns" consist entirely of graphic US-ASCII
    characters, so no translation is needed.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>. Appendix - Example</span>

   This example traces the flow of a single message addressed to
   multiple recipients.  The message is sent by Alice@Pure-Heart.ORG to
   Bob@Big-Bucks.COM, Carol@Ivory.EDU, Dana@Ivory.EDU,
   Eric@Bombs.AF.MIL, Fred@Bombs.AF.MIL, and George@Tax-ME.GOV, with a
   variety of per-recipient options.  The message is successfully
   delivered to Bob, Dana (via a gateway), Eric, and Fred.  Delivery
   fails for Carol and George.

   NOTE: Formatting rules for RFCs require that no line be longer than
   72 characters.  Therefore, in the following examples, some SMTP
   commands longer than 72 characters are printed on two lines, with the
   first line ending in "\".  In an actual SMTP transaction, such a
   command would be sent as a single line (i.e. with no embedded CRLFs),
   and without the "\" character that appears in these examples.

<span class="h3"><a class="selflink" id="section-10.1" href="#section-10.1">10.1</a> Submission</span>

   Alice's user agent sends the message to the SMTP server at Pure-
   Heart.ORG.  Note that while this example uses SMTP as a mail
   submission protocol, other protocols could also be used.

&lt;&lt;&lt; 220 Pure-Heart.ORG SMTP server here
&gt;&gt;&gt; EHLO Pure-Heart.ORG
&lt;&lt;&lt; 250-Pure-Heart.ORG
&lt;&lt;&lt; 250-DSN
&lt;&lt;&lt; 250-EXPN
&lt;&lt;&lt; 250 SIZE
&gt;&gt;&gt; MAIL FROM:&lt;Alice@Pure-Heart.ORG&gt; RET=HDRS ENVID=QQ314159
&lt;&lt;&lt; 250 &lt;Alice@Pure-Heart.ORG&gt; sender ok
&gt;&gt;&gt; RCPT TO:&lt;Bob@Big-Bucks.COM&gt; NOTIFY=SUCCESS \
    ORCPT=<a href="./rfc822">rfc822</a>;Bob@Big-Bucks.COM
&lt;&lt;&lt; 250 &lt;Bob@Big-Bucks.COM&gt; recipient ok
&gt;&gt;&gt; RCPT TO:&lt;Carol@Ivory.EDU&gt; NOTIFY=FAILURE \
    ORCPT=<a href="./rfc822">rfc822</a>;Carol@Ivory.EDU
&lt;&lt;&lt; 250 &lt;Carol@Ivory.EDU&gt; recipient ok
&gt;&gt;&gt; RCPT TO:&lt;Dana@Ivory.EDU&gt; NOTIFY=SUCCESS,FAILURE \
    ORCPT=<a href="./rfc822">rfc822</a>;Dana@Ivory.EDU
&lt;&lt;&lt; 250 &lt;Dana@Ivory.EDU&gt; recipient ok



<span class="grey">Moore                       Standards Track                    [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


&gt;&gt;&gt; RCPT TO:&lt;Eric@Bombs.AF.MIL&gt; NOTIFY=FAILURE \
    ORCPT=<a href="./rfc822">rfc822</a>;Eric@Bombs.AF.MIL
&lt;&lt;&lt; 250 &lt;Eric@Bombs.AF.MIL&gt; recipient ok
&gt;&gt;&gt; RCPT TO:&lt;Fred@Bombs.AF.MIL&gt; NOTIFY=NEVER
&lt;&lt;&lt; 250 &lt;Fred@Bombs.AF.MIL&gt; recipient ok
&gt;&gt;&gt; RCPT TO:&lt;George@Tax-ME.GOV&gt; NOTIFY=FAILURE \
    ORCPT=<a href="./rfc822">rfc822</a>;George@Tax-ME.GOV
&lt;&lt;&lt; 250 &lt;George@Tax-ME.GOV&gt; recipient ok
&gt;&gt;&gt; DATA
&lt;&lt;&lt; 354 okay, send message
&gt;&gt;&gt; (message goes here)
&gt;&gt;&gt; .
&lt;&lt;&lt; 250 message accepted
&gt;&gt;&gt; QUIT
&lt;&lt;&lt; 221 goodbye

<span class="h3"><a class="selflink" id="section-10.2" href="#section-10.2">10.2</a> Relay to Big-Bucks.COM</span>

   The SMTP at Pure-Heart.ORG then relays the message to Big-Bucks.COM.
   (For the purpose of this example, mail.Big-Bucks.COM is the primary
   mail exchanger for Big-Bucks.COM).

&lt;&lt;&lt; 220 mail.Big-Bucks.COM says hello
&gt;&gt;&gt; EHLO Pure-Heart.ORG
&lt;&lt;&lt; 250-mail.Big-Bucks.COM
&lt;&lt;&lt; 250 DSN
&gt;&gt;&gt; MAIL FROM:&lt;Alice@Pure-Heart.ORG&gt; RET=HDRS ENVID=QQ314159
&lt;&lt;&lt; 250 sender okay
&gt;&gt;&gt; RCPT TO:&lt;Bob@Big-Bucks.COM&gt; NOTIFY=SUCCESS \
    ORCPT=<a href="./rfc822">rfc822</a>;Bob@Big-Bucks.COM
&lt;&lt;&lt; 250 recipient okay
&gt;&gt;&gt; DATA
&lt;&lt;&lt; 354 send message
&gt;&gt;&gt; (message goes here)
&gt;&gt;&gt; .
&lt;&lt;&lt; 250 message received
&gt;&gt;&gt; QUIT
&lt;&lt;&lt; 221 bcnu

<span class="h3"><a class="selflink" id="section-10.3" href="#section-10.3">10.3</a> Relay to Ivory.EDU</span>

   The SMTP at Pure-Heart.ORG relays the message to Ivory.EDU, which (as
   it happens) is a gateway to a LAN-based mail system that accepts SMTP
   mail and supports the DSN extension.

&lt;&lt;&lt; 220 Ivory.EDU gateway to FooMail(tm) here
&gt;&gt;&gt; EHLO Pure-Heart.ORG
&lt;&lt;&lt; 250-Ivory.EDU



<span class="grey">Moore                       Standards Track                    [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


&lt;&lt;&lt; 250 DSN
&gt;&gt;&gt; MAIL FROM:&lt;Alice@Pure-Heart.ORG&gt; RET=HDRS ENVID=QQ314159
&lt;&lt;&lt; 250 ok
&gt;&gt;&gt; RCPT TO:&lt;Carol@Ivory.EDU&gt; NOTIFY=FAILURE \
    ORCPT=<a href="./rfc822">rfc822</a>;Carol@Ivory.EDU
&lt;&lt;&lt; 550 error - no such recipient
&gt;&gt;&gt; RCPT TO:&lt;Dana@Ivory.EDU&gt; NOTIFY=SUCCESS,FAILURE \
    ORCPT=<a href="./rfc822">rfc822</a>;Dana@Ivory.EDU
&lt;&lt;&lt; 250 recipient ok
&gt;&gt;&gt; DATA
&lt;&lt;&lt; 354 send message, end with '.'
&gt;&gt;&gt; (message goes here)
&gt;&gt;&gt; .
&lt;&lt;&lt; 250 message received
&gt;&gt;&gt; QUIT
&lt;&lt;&lt; 221 bye

   Note that since the Ivory.EDU refused to accept mail for
   Carol@Ivory.EDU, and the sender specified NOTIFY=FAILURE, the
   sender-SMTP (in this case Pure-Heart.ORG) must generate a DSN.

<span class="h3"><a class="selflink" id="section-10.4" href="#section-10.4">10.4</a> Relay to Bombs.AF.MIL</span>

   The SMTP at Pure-Heart.ORG relays the message to Bombs.AF.MIL, which
   does not support the SMTP extension.  Because the sender specified
   NOTIFY=NEVER for recipient Fred@Bombs.AF.MIL, the SMTP at Pure-
   Heart.ORG chooses to send the message for that recipient in a
   separate transaction with a reverse-path of &lt;&gt;.

&lt;&lt;&lt; 220-Bombs.AF.MIL reporting for duty.
&lt;&lt;&lt; 220 Electronic mail is to be used for official business only.
&gt;&gt;&gt; EHLO Pure-Heart.ORG
&lt;&lt;&lt; 502 command not implemented
&gt;&gt;&gt; RSET
&lt;&lt;&lt; 250 reset
&gt;&gt;&gt; HELO Pure-Heart.ORG
&lt;&lt;&lt; 250 Bombs.AF.MIL
&gt;&gt;&gt; MAIL FROM:&lt;Alice@Pure-Heart.ORG&gt;
&lt;&lt;&lt; 250 ok
&gt;&gt;&gt; RCPT TO:&lt;Eric@Bombs.AF.MIL&gt;
&lt;&lt;&lt; 250 ok
&gt;&gt;&gt; DATA
&lt;&lt;&lt; 354 send message
&gt;&gt;&gt; (message goes here)
&gt;&gt;&gt; .
&lt;&lt;&lt; 250 message accepted
&gt;&gt;&gt; MAIL FROM:&lt;&gt;
&lt;&lt;&lt; 250 ok



<span class="grey">Moore                       Standards Track                    [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


&gt;&gt;&gt; RCPT TO:&lt;Fred@Bombs.AF.MIL&gt;
&lt;&lt;&lt; 250 ok
&gt;&gt;&gt; DATA
&lt;&lt;&lt; 354 send message
&gt;&gt;&gt; (message goes here)
&gt;&gt;&gt; .
&lt;&lt;&lt; 250 message accepted
&gt;&gt;&gt; QUIT
&lt;&lt;&lt; 221 Bombs.AF.MIL closing connection

<span class="h3"><a class="selflink" id="section-10.5" href="#section-10.5">10.5</a> Forward from George@Tax-ME.GOV to Sam@Boondoggle.GOV</span>

   The SMTP at Pure-Heart.ORG relays the message to Tax-ME.GOV.  (this
   step is not shown).  MTA Tax-ME.GOV then forwards the message to
   Sam@Boondoggle.GOV (shown below).  Both Tax-ME.GOV and Pure-Heart.ORG
   support the SMTP DSN extension.  Note that RET, ENVID, and ORCPT all
   retain their original values.

&lt;&lt;&lt; 220 BoonDoggle.GOV says hello
&gt;&gt;&gt; EHLO Pure-Heart.ORG
&lt;&lt;&lt; 250-mail.Big-Bucks.COM
&lt;&lt;&lt; 250 DSN
&gt;&gt;&gt; MAIL FROM:&lt;Alice@Pure-Heart.ORG&gt; RET=HDRS ENVID=QQ314159
&lt;&lt;&lt; 250 sender okay
&gt;&gt;&gt; RCPT TO:&lt;Sam@Boondoggle.GOV&gt; NOTIFY=SUCCESS \
    ORCPT=<a href="./rfc822">rfc822</a>;George@Tax-ME.GOV
&lt;&lt;&lt; 250 recipient okay
&gt;&gt;&gt; DATA
&lt;&lt;&lt; 354 send message
&gt;&gt;&gt; (message goes here)
&gt;&gt;&gt; .
&lt;&lt;&lt; 250 message received
&gt;&gt;&gt; QUIT
&lt;&lt;&lt; 221 bcnu

















<span class="grey">Moore                       Standards Track                    [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


<span class="h3"><a class="selflink" id="section-10.6" href="#section-10.6">10.6</a> "Delivered" DSN for Bob@Big-Bucks.COM</span>

   MTA mail.Big-Bucks.COM successfully delivers the message to Bob@Big-
   Bucks.COM.  Because the sender specified NOTIFY=SUCCESS, mail.Big-
   Bucks.COM issues the following DSN, and sends it to Alice@Pure-
   Heart.ORG.

To: Alice@Pure-Heart.ORG
From: postmaster@mail.Big-Bucks.COM
Subject: Delivery Notification (success) for Bob@Big-Bucks.COM
Content-Type: multipart/report; report-type=delivery-status;
    boundary=abcde
MIME-Version: 1.0

--abcde
Content-type: text/plain; charset=us-ascii

Your message (id QQ314159) was successfully delivered to
Bob@Big-Bucks.COM.

--abcde
Content-type: message/delivery-status

Reporting-MTA: dns; mail.Big-Bucks.COM
Original-Envelope-ID: QQ314159

Original-Recipient: <a href="./rfc822">rfc822</a>;Bob@Big-Bucks.COM
Final-Recipient: <a href="./rfc822">rfc822</a>;Bob@Big-Bucks.COM
Action: delivered
Status: 2.0.0

--abcde
Content-type: message/rfc822

(headers of returned message go here)

--abcde--














<span class="grey">Moore                       Standards Track                    [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


<span class="h3"><a class="selflink" id="section-10.7" href="#section-10.7">10.7</a> Failed DSN for Carol@Ivory.EDU</span>

   Because delivery to Carol failed and the sender specified
   NOTIFY=FAILURE for Carol@Ivory.EDU, MTA Pure-Heart.ORG (the SMTP
   client to which the failure was reported via SMTP) issues the
   following DSN.

To: Alice@Pure-Heart.ORG
From: postmaster@Pure-Heart.ORG
Subject: Delivery Notification (failure) for Carol@Ivory.EDU
Content-Type: multipart/report; report-type=delivery-status;
              boundary=bcdef
MIME-Version: 1.0

--bcdef
Content-type: text/plain; charset=us-ascii

Your message (id QQ314159) could not be delivered to
Carol@Ivory.EDU.

A transcript of the session follows:

(while talking to Ivory.EDU)
&gt;&gt;&gt; RCPT TO:&lt;Carol@Ivory.EDU&gt; NOTIFY=FAILURE
&lt;&lt;&lt; 550 error - no such recipient

--bcdef
Content-type: message/delivery-status

Reporting-MTA: dns; Pure-Heart.ORG
Original-Envelope-ID: QQ314159

Original-Recipient: <a href="./rfc822">rfc822</a>;Carol@Ivory.EDU
Final-Recipient: <a href="./rfc822">rfc822</a>;Carol@Ivory.EDU
SMTP-Remote-Recipient: Carol@Ivory.EDU
Diagnostic-Code: smtp; 550 error - no such recipient
Action: failed
Status: 5.0.0

--bcdef
Content-type: message/rfc822

(headers of returned message go here)

--bcdef--






<span class="grey">Moore                       Standards Track                    [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


<span class="h3"><a class="selflink" id="section-10.8" href="#section-10.8">10.8</a> Relayed DSN For Dana@Ivory.EDU</span>

   Although the mail gateway Ivory.EDU supports the DSN SMTP extension,
   the LAN mail system attached to its other side does not generate
   positive delivery confirmations.  So Ivory.EDU issues a "relayed"
   DSN:

To: Alice@Pure-Heart.ORG
From: postmaster@Ivory.EDU
Subject: mail relayed for Dana@Ivory.EDU
Content-Type: multipart/report; report-type=delivery-status;
    boundary=cdefg
MIME-Version: 1.0

--cdefg
Content-type: text/plain; charset=us-ascii

Your message (addressed to Dana@Ivory.EDU) was successfully
relayed to:

ymail!Dana

by the FooMail gateway at Ivory.EDU.

Unfortunately, the remote mail system does not support
confirmation of actual delivery.  Unless delivery to ymail!Dana
fails, this will be the only delivery status notification sent.

--cdefg
Content-type: message/delivery-status

Reporting-MTA: dns; Ivory.EDU
Original-Envelope-ID: QQ314159

Original-Recipient: <a href="./rfc822">rfc822</a>;Dana@Ivory.EDU
Final-Recipient: <a href="./rfc822">rfc822</a>;Dana@Ivory.EDU
Action: relayed
Status: 2.0.0

--cdefg
Content-type: message/rfc822

(headers of returned message go here)

--cdefg--






<span class="grey">Moore                       Standards Track                    [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


<span class="h3"><a class="selflink" id="section-10.9" href="#section-10.9">10.9</a> Failure notification for Sam@Boondoggle.GOV</span>

   The message originally addressed to George@Tax-ME.GOV was forwarded
   to Sam@Boondoggle.GOV, but the MTA for Boondoggle.GOV was unable to
   deliver the message due to a lack of disk space in Sam's mailbox.
   After trying for several days, Boondoggle.GOV returned the following
   DSN:

To: Alice@BigHeart.ORG
From: Postmaster@Boondoggle.GOV
Subject: Delivery failure for Sam@Boondoggle.GOV
Content-Type: multipart/report; report-type=delivery-status;
              boundary=defgh
MIME-Version: 1.0

--defgh
Your message, originally addressed to George@Tax-ME.GOV, and forwarded
from there to Sam@Boondoggle.GOV could not be delivered, for the
following reason:

write error to mailbox, disk quota exceeded

--defgh
Content-type: message/delivery-status

Reporting-MTA: Boondoggle.GOV
Original-Envelope-ID: QQ314159

Original-Recipient: <a href="./rfc822">rfc822</a>;George@Tax-ME.GOV
Final-Recipient: <a href="./rfc822">rfc822</a>;Sam@Boondoggle.GOV
Action: failed
Status: 4.2.2 (disk quota exceeded)

--defgh
Content-type: message/rfc822

(headers of returned message go here)

--defgh--












<span class="grey">Moore                       Standards Track                    [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc1891">RFC 1891</a>           SMTP Delivery Status Notifications       January 1996</span>


<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>. References</span>

   [<a id="ref-1">1</a>] Postel, J., "Simple Mail Transfer Protocol", STD 10, <a href="./rfc821">RFC 821</a>,
       USC/Information Sciences Institute, August 1982.

   [<a id="ref-2">2</a>] Crocker, D., "Standard for the Format of ARPA Internet Text
       Messages", STD 11, <a href="./rfc822">RFC 822</a>, UDEL, August 1982.

   [<a id="ref-3">3</a>] Westine, A., and J. Postel, "Problems with the Maintenance of
       Large Mailing Lists.", <a href="./rfc1211">RFC 1211</a>, USC/Information Sciences
       Institute, March 1991.

   [<a id="ref-4">4</a>] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker,
       "SMTP Service Extensions", <a href="./rfc1651">RFC 1651</a>, MCI, Innosoft, Dover Beach
       Consulting, Inc., Network Management Associates, Inc., Silicon
       Graphics, Inc., July 1994.

   [<a id="ref-5">5</a>] Moore, K., and G. Vaudreuil, "An Extensible Message Format for
       Delivery Status Notifications", <a href="./rfc1894">RFC 1894</a>, University of Tennessee,
       Octel Network Services, January 1996.

   [<a id="ref-6">6</a>] Crispin, M., "Internet Message Access Protocol - Version 4", <a href="./rfc1730">RFC</a>
       <a href="./rfc1730">1730</a>, University of Washington, 20 December 1994.

   [<a id="ref-7">7</a>] Myers, J., and M. Rose, "Post Office Protocol - Version 3", <a href="./rfc1725">RFC</a>
       <a href="./rfc1725">1725</a>, Carnegie Mellon, Dover Beach Consulting, November 1994.

   [<a id="ref-8">8</a>] Vaudreuil, G., "The Multipart/Report Content Type for the
       Reporting of Mail System Administrative Messages", <a href="./rfc1892">RFC 1892</a>, Octel
       Network Services, January 1996.

   [<a id="ref-9">9</a>] Braden, R., Editor, "Requirements for Internet Hosts - Application
       and Support", STD 3, <a href="./rfc1123">RFC 1123</a>, IETF, October 1989.

   [<a id="ref-10">10</a>] Vaudreuil, G., "Enhanced Mail System Status Codes", <a href="./rfc1893">RFC 1893</a>,
        Octel Network Services, January 1996.

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>. Author's Address</span>

   Keith Moore
   University of Tennessee
   107 Ayres Hall
   Knoxville, TN 37996-1301
   USA

   EMail: moore@cs.utk.edu





Moore                       Standards Track                    [Page 31]
</pre>