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 result in the
generation of DSNs.
DISCUSSION: RFC 1123, section 2.3.3 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.
6.2.1 Relay of messages to other conforming SMTP servers
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.
6.2.2 Relay of messages to non-conforming SMTP servers
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.
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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 [1].
(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.
6.2.3 Local delivery of messages
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 [6], POP [7] 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.
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6.2.4 Gatewaying a message into a foreign environment
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) I