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Keywords: handling, email, mhs, recipient, transport, delivery, mda, mta, relay, header

Independent Submission D. Crocker, Ed.
Request for Comments: 9228 Brandenburg InternetWorking
Category: Experimental April 2022
ISSN: 2070-1721
Delivered-To Email Header Field
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 email's author. For example, the address used by the
email transport service is provided separately, such as through
SMTP's "RCPT TO" command, and might not match any address in the To:
or cc: fields. In addition, before final delivery, handling can
entail a sequence of submission/delivery events, using a sequence of
different destination addresses that (eventually) lead to the
recipient. As well, a receiving system's delivery process can
produce local address transformations.
It can be helpful for a message to have a common way to record each
delivery in such a sequence, noting each address used in the sequence
to that recipient, such as for (1) analyzing the path a message has
taken, (2) loop detection, or (3) formulating the author's address in
a reply message. This document defines a header field for this
information.
Email handling information discloses details about the email
infrastructure, as well as about a particular recipient; this can
raise privacy concerns.
A header field such as this is not automatically assured of
widespread use. Therefore, this document is being published as an
Experimental RFC, looking for constituency and for operational
utility. This document was produced through the Independent
Submission Stream and was not subject to the IETF's approval process.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9228.
Copyright Notice
Copyright (c) 2022 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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction
2. Background
3. Framework & Terminology
4. Delivered-To
5. Multi-Delivery Example
6. Security Considerations
7. IANA Considerations
8. Experimental Goals
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Author's Address
1. Introduction
The address to which email is delivered might be different than any
of the addresses shown in any of the content header fields
[Mail-Fmt], such as the To: and cc: fields that were created by the
author's Message User Agent (MUA) [Mail-Arch]. The address used by
the Message Handling Service (MHS) is provided separately, in
envelope information, such as through a "RCPT TO" command in [SMTP].
As noted in Section 4.3.3 of [Mail-Arch], 'A transfer of
responsibility from the MHS to a Recipient's environment (mailbox) is
called "delivery".' That is, when the destination address is fully
and successfully processed, and any additional processing is by an
agent working on behalf of that address, the message has been
delivered. Rather than placing the message into a recipient inbox or
otherwise completing the handling of the message, that agent might
create additional processing, including to one or more different
addresses. Each transition of responsibility, from the MHS to an
agent of a current addressee, constitutes a distinct delivery. Given
handling sequences that can include 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. Also, the
delivery process at a receiving system can produce local (internal)
address transformations.
Header fields that provide information about handling can be used
when assessing email traffic issues and when diagnosing specific
handling problems. To this end, it can be helpful for a message to
have a common way to indicate each delivery in the handling sequence
and to include each address that led to the final delivery. This can
aid in the analysis of a message's transit handling.
An additional use can be to aid in detecting a delivery sequence
loop, based on a specific address. With a problematic loop, the same
copy of a message is delivered to the same email address more than
once. This is different from having different copies delivered to
the same address, such as happens when a message is sent directly to
an address, as well as via a mailing list. It is also different from
having two copies of the same message arrive at the same, ultimate
destination address, having been originally posted to two different
addresses. Further, this is different from noting when a message
simply transits the same Message Transfer Agent (MTA) more than once,
which might be necessary, such as when it is processed through a
mailing list; an MTA services many addresses.
Delivering the same copy of a message more than once, to the same
address, is almost certainly not an intended activity. An example of
a problematic arrangement would be to send a message to mailing list
List-A, where List-A contains an entry for List-B, and List-B
contains an entry for List-A. The message will enter an infinite
loop. Loop detection for email can be a complicated affair. The
Delivered-To: header field provides helpful information, with a
definitive indication that this copy of a message has (already) been
delivered to a specific address.
When specifying new activity that is related to existing activity,
there is a choice of design approach:
* Seeking to change (some of) the existing behavior
* Adding to the activity without changing what is already being done
* Calling for separate, new activity
On the average, attempting to change existing activities is the least
likely to obtain adoption; it can create operational confusion
between old and new activities, which in turn creates resistance to
adoption. Seeking new activity can make sense when that activity is
sufficiently different and deemed sufficiently beneficial. Adding to
existing activity has the selling point of building upon an installed
base. The current specification builds upon an existing installed
base of Delivered-To: activity. It calls for little technical
enhancement; rather, it simply provides for a wider range of
application.
Considerations:
* Email handling information, such as this, provides information
about the email infrastructure, as well as about the recipient.
Disclosure of this information might engender privacy concerns.
* A specification is not automatically assured of adoption or use.
Therefore, this document is being published as an Experimental
RFC, looking for extended constituency and for general operational
utility.
* This document was produced through the Independent RFC Stream and
was not subject to the IETF's approval process.
2. Background
Ad hoc use of a Delivered-To: email header field appears to date back
to the 1990s, primarily for loop detection, although documentation is
spotty and system specific. A listing of some implementations is
offered in [Prior].
It appears that all uses include a string in the form of an email
address, although at least one example has leading text that is a
comment about the address. In some cases, the string appears to be
the email transport destination address, such as the address used in
SMTP's "RCPT TO" command. In other cases, it appears to be the
result of some internal mapping at the receiving system, although
tending to be a variant of the transport address.
Email loop detection tends to be accomplished through a variety of
different methods, such as counting Received: header fields. These
methods are often combined to greater effect.
The Received: header field's 'for' clause is sometimes useful for
disclosing the recipient's address. However, the clause is not used
reliably, and its semantics are not thoroughly defined. Also, it
references an addressing value that is received but might be
different from the value that is ultimately used (as the result of a
transformation). That is, the value in a 'for' clause might be a
sufficient indicator of delivery addressing, but it might not.
3. Framework & Terminology
Unless otherwise indicated, basic architecture and terminology used
in this document are taken from:
* [Mail-Arch]
* [SMTP]
* [Mail-Fmt]
and syntax is specified with:
* [ABNF]
Normative language is 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.
4. Delivered-To
The Delivered-To: header field annotates an email delivery event.
The header field contains information about the individual address
used to effect that transition.
* When a message is delivered, as a transition from control by the
MHS to the recipient's store or their agent, a Delivered-To:
header field SHOULD be added, with the _addr-spec_ value
containing the address that was used by the service to reach the
recipient.
* If a receiving system's delivery process applies mappings or
transformations from the address used by the MHS to a local value,
this new value SHOULD also be recorded into a separate Delivered-
To: field when transit and processing using that address
successfully complete. This ensures a detailed record of the
sequence of handling addresses used for the message.
* As with some other information, each additional Delivered-To:
header field MUST be placed at the current 'top' of the message's
set of header fields -- that is, as the first header field, in a
fashion similar to the trace fields specified in [SMTP] (for
example, Section 4.1.1.4 of [SMTP]). This produces a sequence of
Delivered-To: header fields that represent the sequence of
deliveries, with the first being at the 'bottom' of the sequence
and the final one being at the top.
* As with other fields placed incrementally in this way, with each
added at the current top, the Delivered-To: header field MUST NOT
be reordered with respect to other Delivered-To: fields and those
other fields. This is intended to preserve the fields as
representing the message handling sequence.
The Delivered-To: header field is added at the time of delivery, when
responsibility for a message transitions from the Message Handling
Service (MHS) to an agent of the specified individual recipient
address. The field can also be added as a result of internal system
processing, to note address transformations.
| Note: The presence of an existing Delivered-To: header field,
| for the same address, typically indicates a handling loop for
| this instance of the message.
The syntax of the header field is:
"Delivered-To:" FWS addr-spec FWS CRLF ; addr-spec is from [Mail-Fmt]
The field records information about a single address, for one
recipient. See Section 6 for the privacy-related concerns about
divulging addresses.
5. Multi-Delivery Example
The Delivered-To: header field can be used to document a sequence of
deliveries of a message. Each time an address is fully processed, a
Delivered-To: header field is added, recording a handling sequence,
with the most recent one being towards the 'top' of the sequence of
header fields.
This example demonstrates a message traveling from its original
posting, through a remote group mailing list, on through an
independent personal aliasing mechanism, and then reaching final
delivery at yet another independent email provider.
1. Origination at com.example
The message, as submitted. The destination address is the
same as the value in the message's To: header field.
From: Ann Author <aauthor@com.example>
Date: Mon, 25 Jan 2021 18:29:06 -0500
To: list@org.example
Subject: [list] Sending through a list and alias
Sender: Ann Author <aauthor@com.example>
2. List processing at org.example
As delivered, with one Delivered-To: header field, to the list
processing module, which will then resubmit the message for
further transport to the list member "Recipient-
alumn@edu.example".
Delivered-To: list@org.example
Received: by submit.org.example with SMTP id i17so17480689ljn.1
for <list@org.example> from mail.com.example;
Mon, 25 Jan 2021 15:29:19 -0800 (PST)
Received: by mail.com.example; Mon, 25 Jan 2021 15:29:00 -0800 (PST)
From: Ann Author <aauthor@com.example>
Date: Mon, 25 Jan 2021 18:29:06 -0500
To: list@org.example
Subject: [list] Sending through a list and alias
Sender: Ann Author <aauthor@com.example>
3. Alias processing at edu.example
The message, as delivered with two Delivered-To: header
fields, to the alias processing module, which sends the
message on to "theRecipient@example.net".
Delivered-To: Recipient-alumn@edu.example
Received: from mail.org.example
by relay.edu.example; Mon, 25 Jan 2021 23:29:24 +0000 (UTC)
Received: by submit.org.example;
Mon, 25 Jan 2021 23:29:21 +0000 (UTC)
Delivered-To: list@org.example
Received: by submit.org.example with SMTP id i17so17480689ljn.1
for <list@org.example> from mail.com.example;
Mon, 25 Jan 2021 15:29:19 -0800 (PST)
Received: by mail.com.example; Mon, 25 Jan 2021 15:29:00 -0800 (PST)
From: Ann Author <aauthor@com.example>
Date: Mon, 25 Jan 2021 18:29:06 -0500
To: list@org.example
Subject: [list] Sending through a list and alias
Sender: list-bounces@org.example
4. Final delivery to the recipient at example.net
The message, as finally delivered with three Delivered-To:
header fields, to the recipient at "theRecipient@example.net".
Delivered-To: theRecipient@example.net
Received: from mail.edu.example (mail.edu.example [4.31.198.45])
by relay.example.net; Mon, 25 Jan 2021 23:29:24 +0000 (UTC)
Delivered-To: Recipient-alumn@edu.example
Received: from mail.org.example
by relay.edu.example; Mon, 25 Jan 2021 23:29:24 +0000 (UTC)
Received: by submit.org.example;
Mon, 25 Jan 2021 23:29:21 +0000 (UTC)
Delivered-To: list@org.example
Received: by submit.org.example with SMTP id i17so17480689ljn.1
for <list@org.example> from mail.com.example;
Mon, 25 Jan 2021 15:29:19 -0800 (PST)
Received: by mail.com.example; Mon, 25 Jan 2021 15:29:00 -0800 (PST)
From: Ann Author <aauthor@com.example>
Date: Mon, 25 Jan 2021 18:29:06 -0500
To: list@org.example
Subject: [list] Sending through a list and alias
Sender: list-bounces@org.example
6. Security Considerations
As with Received: header fields, the presence of a Delivered-To:
header field discloses handling information and, possibly, personal
information.
Security and privacy are essential, if challenging, topics for email
in general and for the handling of metadata in particular. The
purpose of this section is to note points of potential concern,
rather than to provide details for mitigation. The basic mechanism
described here has a long history of use, with no history of being
problematic. However, the expanded use described here might create
new scenarios that are problematic.
An issue specific to this mechanism is disclosure of a sequence of
addresses, applied to the same recipient, if a message goes through a
series of recipient address replacements. This document calls for
each of these addresses to be recorded in a separate Delivered-To:
field. This does not disclose addresses of other recipients, but it
does disclose an address-transformation handling path for the
recipient.
This disclosure is most likely to be a concern when a recipient
manually forwards a message and includes all of the original header
fields. This will expose, to a later recipient, any intermediate
addresses used for getting the original message to the original
recipient. Such a disclosure is likely to be unintended and might be
(highly) problematic. Note that a basic version of this unintended
disclosure has long existed, by virtue of a later recipient's seeing
Received: header fields, but especially any with a 'for' clause.
However, a Delivered-To: header field sequence can disclose
significantly more recipient-specific handling detail.
An issue that is entirely implementation specific -- and therefore
out of scope for this document -- is that in some systems, a message
that is for multiple (local) recipients is stored as a single, shared
version. Supporting Delivered-To:, while maintaining recipient
privacy, creates a challenge in this case, since exposing different
recipient addresses to other recipients can be problematic.
7. IANA Considerations
IANA has registered the Delivered-To: header field as below, per
[RFC3864] in the "Provisional Message Header Field Names" registry:
Header Field Name: Delivered-To
Protocol: mail
Status: Provisional
Author/Change controller: Dave Crocker
Specification document(s): *** This document ***
Related information: None.
8. Experimental Goals
Specific feedback is sought concerning:
* Technical issues in recording the Delivered-To: field into a
message, through its entire submission/delivery sequence
* Market interest in the uses described here
* Utility for the purposes described here, or for other uses
So the questions to answer for this Experimental RFC are:
* Is there demonstrated interest by MSA/MTA/MDA (Message Submission
Agent / Message Transfer Agent / Message Delivery Agent)
developers?
* If the capability is implemented and the header field generated,
is it used by operators or MUAs?
* Does the presence of the header field create any operational
problems?
* Does the presence of the header field demonstrate additional
security issues?
* What specific changes to the document are needed?
* What other comments will aid in use of this mechanism?
Please send comments to ietf-smtp@ietf.org.
9. References
9.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[Mail-Arch]
Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.
[Mail-Fmt] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[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>.
9.2. Informative References
[Prior] Dukhovni, V. and J. Levine, "The Delivered-To Message
Header Field", Work in Progress, Internet-Draft, draft-
duklev-deliveredto-01, 6 February 2022,
<https://datatracker.ietf.org/doc/html/draft-duklev-
deliveredto-01>.
Acknowledgements
Even a simple, narrow specification can elicit a remarkable range and
intensity of debate. In spite of the current document's being a case
of that challenge, useful discussion has taken place, first in the
IETF's emailcore working group mailing list, and then on the long-
standing ietf-smtp mailing list.
Helpful information and suggestions were provided by Anonymous,
Stéphane Bortzmeyer, Richard Clayton, Viktor Dukhovni, Adrian Farrel,
Ned Freed, John Klensin, Barry Leiba, Brandon Long, George
Michaelson, Michael Peddemors, Phil Pennock, Pete Resnick, Sam
Varshavchik, Alessandro Vesely, and Tim Wicinski.
Author's Address
Dave Crocker (editor)
Brandenburg InternetWorking
Email: dcrocker@bbiw.net