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 . Requests to subscribe to the mailing list should be addressed to . 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 Moore Standards Track [Page 1] RFC 1891 SMTP Delivery Status Notifications January 1996 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]. Moore Standards Track [Page 2] RFC 1891 SMTP Delivery Status Notifications January 1996 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 Moore Standards Track [Page 3] RFC 1891 SMTP Delivery Status Notifications January 1996 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. Moore Standards Track [Page 4] RFC 1891 SMTP Delivery Status Notifications January 1996 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, Moore Standards Track [Page 5] RFC 1891 SMTP Delivery Status Notifications January 1996 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: Moore Standards Track [Page 6] RFC 1891 SMTP Delivery Status Notifications January 1996 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: