| Internet-Draft | react | December 2021 |
| Crocker | Expires 24 June 2022 | [Page] |
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, and to note each address used in the sequence to that recipient, such as for analyzing the path a message has taken, or for loop detection, or for 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 is being published as an Experiment, looking for constituency and for operational utility. The document is produced through the Independent RFC stream and was not subject to the IETF's approval process.¶
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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 Mail User Agent (MUA) [Mail-Arch]. The address used by the Message Handling Service (MHS) is provided separately, in envelope information, such as through an "RCPT TO" command in [SMTP].¶
Delivery is a transition of responsibility for a message, from the MHS, over to an agent of the destination, as represented by the envelope address (Section 4.3.3 [Mail-Arch]). 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 of indicating 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 as be an aid in detecting a delivery sequence loop. With a loop, the same copy of a message transits the same email address more than once. This is different from having the message simply transit the same MTA more than once, which might be necessary, such as when it is processed through a mailing list; an MTA services many addresses. It is also different from having two copies of the same message arrive at the same, ultimate destination, having been originally posted to two different 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:¶
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, but rather, it simply provides for wider range of application.¶
Considerations:¶
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 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.¶
Unless otherwise indicated, basic architecture and terminology used in this document are taken from:¶
and syntax is specified with:¶
The "Delivered-To:" header field annotates an email delivery event. The header field contains information about the individual address used to effect that transition.¶
The Delivered-To: header field is added at the time of delivery, when responsibility for a message transitions from the Message Handling (Transport) 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.¶
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.¶
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 towards the 'top' of the list of header fields.¶
This example demonstrates a message traveling from 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.¶
Origination @ com.example¶
From: Ann Author <aauthor@com.example> Date: Mon, 25 Jan 2021 18:29:00 -0500 To: list@org.example Subject: [list] Sending through a list and alias Sender: Ann Author <aauthor@com.example>¶
List @ org.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>¶
Alias @ edu.example¶
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¶
Delivery @ 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¶
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, for the same recipient, if a message goes through a series of recipient address modifications. The 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 a address-transformation handling path for the recipient.¶
Where this disclosure is most likely to be a concern is when a recipient manually forwards a message and includes all of the original header fields. This will expose 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 already exists, by virtue of a later recipient 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 to 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.¶
Registration of the "Delivered-To:" header field is requested, per [RFC3864]:¶
Specific feedback is sought concerning:¶
So the questions to answer for this Experimental document are:¶
Please send comments to ietf-smtp@ietf.org.¶
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, Richard Clayton, Viktor Dukhovni, Adrian Farrel, Ned Freed, John Klensin, John Levine, Barry Leiba, Brandon Long, George Michaelson, Michael Peddemors, Phil Pennock, Pete Resnick, Sam Varshavchik, Alessandro Vesely, Tim Wicinski.¶