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Network Working Group D. Crocker, Ed.
Internet-Draft Brandenburg InternetWorking
Intended status: Standards Track February 3, 2021
Expires: August 7, 2021
Delivered-To Email Header Field
draft-crocker-email-deliveredto-00
Abstract
The address to which email is delivered might be different than any
of the addresses shown in any of the content header fields that were
created by the author. The address used by the mail transport
service is provided separately, through an envelope SMTP "RCPT TO"
command. Before final delivery, handling can entail a sequence of
addresses that lead to the recipient. It can be helpful for a
message to have a common way to record each delivery in such a
sequence, and to include each address used for that recipient. This
specification defines a header field for this information.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 7, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Framework & Terminology . . . . . . . . . . . . . . . . . . . 2
3. Delivered-To: . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Multi-hop Example . . . . . . . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The address to which email is delivered might be different than any
of the addresses shown in any of the [Mail-Fmt] content header fields
that were created by the author's Mail User Agent (MUA). The address
used by the Message Handling Service (MHS) [Mail-Arch] is provided
separately, through an envelope SMTP "RCPT TO" command [SMTP].
Delivery is the final processing of an envelope address, with a
transition of responsibility from the MHS, over to an agent
responsible for that address (Section 4.3.3 [Mail-Arch]). After this
transition there might be further, fresh processing of the message,
before reaching a final destination. Each transition of
responsibility, from the MHS to an agent acting on behalf of the
envelope address, constitutes a delivery.
Given aliasing, mailing lists, and the like, the transit of a message
from its author to a final recipient might include a series of
submission/delivery events. It can be helpful for a message to have
a common way to record each delivery in such a sequence, and to
include each address that led to the final delivery.
2. Framework & Terminology
Unless otherwise indicated, basic architecture and terminology used
in this specification are taken from:
o [Mail-Arch]
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o [SMTP]
o [Mail-Fmt]
and syntax is specified with:
o [ABNF]
Normative language, per [RFC8174]:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
appear in all capitals, as shown here.
Discussion of this draft is best conducted in the: ietf-smtp [1]
mailing list.
3. Delivered-To:
This specification defines the "Delivered-To" trace header field, for
annotating a delivery event and the address to which delivery was
effected. A sequence of deliveries, such as when a message goes
through multiple mailing lists, SHOULD be recorded with a series of
Delivered-To: header fields. As with other trace information, each
additional Delivered-To header field MUST be placed at the 'top' of
the current message, per Section 4.1.1.4 [SMTP].
The Delivered-To: header field is added at the time of delivery, when
responsibility for a message transitions from the Mail Handling
(Transport) Service to an agent acting on behalf of the specified
recipient address. The header field contains the individual address
used to determine the immediate location for that transition.
Syntax of the header field is:
"Delivered-To:" FWS Mailbox CRLF
Note: The field records only a single address, for one recipient.
4. Multi-hop Example
Sending through a mailing list and on through an alias, traveling:
1. Origination @ example.com
2. List @ example.org
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Delivered-To: list@example.org
Received: by submit.example.org with SMTP id i17so17480689ljn.1
for <list@example.org>; Mon, 25 Jan 2021 15:29:19 -0800 (PST)
From: Ann Author <aauthor@example.com>
Date: Mon, 25 Jan 2021 18:29:06 -0500
To: list@example.org
Subject: [list] Sending through a list and alias
Sender: Ann Author <aauthor@example.com>
3. Alias @ example.edu
Delivered-To: Recipient-alumn@example.edu
Received: from mail.example.org
by relay.example.edu; Mon, 25 Jan 2021 23:29:24 +0000 (UTC)
Delivered-To: list@example.org
Received: by submit.example.org with SMTP id i17so17480689ljn.1
for <list@example.org>; Mon, 25 Jan 2021 15:29:19 -0800 (PST)
From: Ann Author <aauthor@example.com>
Date: Mon, 25 Jan 2021 18:29:06 -0500
To: list@example.org
Subject: [list] Sending through a list and alias
Sender: list-bounces@example.org
4. Delivery @ example.net
Delivered-To: theRecipient@example.net
Received: from mail.example.edu (mail.example.edu [4.31.198.45])
by relay.example.net; Mon, 25 Jan 2021 23:29:24 +0000 (UTC)
Delivered-To: Recipient-alumn@example.edu
Received: from mail.example.org
by relay.example.edu; Mon, 25 Jan 2021 23:29:24 +0000 (UTC)
Delivered-To: list@example.org
Received: by submit.example.org with SMTP id i17so17480689ljn.1
for <list@example.org>; Mon, 25 Jan 2021 15:29:19 -0800 (PST)
From: Ann Author <aauthor@example.com>
Date: Mon, 25 Jan 2021 18:29:06 -0500
To: list@example.org
Subject: [list] Sending through a list and alias
Sender: list-bounces@example.org
5. Security Considerations
As with Received header-fields, their presence in a message discloses
handling information and, possibly, personal information.
Delivering one stored message to multiple recipients creates a
challenge for representing the separate, individual Delivered-To
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fields -- one for each actual recipient -- without, exposing the
different mailbox names to each other; such exposure MUST NOT occur.
An issue specific to this mechanism is disclosure of a sequence of
addresses, if a message goes through a series of recipient envelope
address modifications. The specification calls for each of these
addresses to be recorded in separate Delivered-To: fields. This does
not disclose addresses of other, possible recipients, but it does
disclose a address-transformation handling path for the recipient.
6. IANA Considerations
Registration of the "Delivered-To" header field is requested, per
[RFC3864]:
Header field name:: Delivered-To
Applicable protocol:: mail
Status:: Standard
Author/Change controller:: IETF
Specification document(s): *** This document ***
Related information: None.
7. References
7.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008.
[Mail-Arch]
Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009.
[Mail-Fmt]
Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
7.2. URIs
[1] mailto:ietf-smtp@ietf.org
Appendix A. Acknowledgements
This specification was engendered by discussions in the IETF's
emailcore working group, although the result was outside the scope of
its charter.
Helpful comments were provided by Ned Freed, Barry Leiba, John
Levine, Brandon Long, Michael Peddemors, Phil Pennock.
Author's Address
Dave Crocker (editor)
Brandenburg InternetWorking
Email: dcrocker@bbiw.net
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