rfc1891.txt

用C#开发实现SMTP相关技术,能接收到带附件的邮件服务功能.

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
   <[e-mail]>.  Requests to subscribe to the mailing
   list should be addressed to <[e-mail]>.
   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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