| Internet-Draft | react | July 2021 |
| Crocker | Expires 31 January 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 author. The address used by the email transport service is provided separately, through an envelope SMTP "RCPT TO" command. Before final delivery, handling can entail a sequence of submission/delivery events, using different destination 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 document defines a header field for this information.¶
Email handling information is a matter of modifying the email infrastructure and possibly creating privacy concerns. A header field such as this is not automatically assured of adoption or use. Therefore it is being published as an Experiment, looking for constituency and for operational utility.¶
This 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] 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, through an envelope "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 of the addressee, 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 of indicating each delivery in the handling sequence, and to include each address that led to the final delivery. One use can be for detecting a delivery sequence loop. Delivering the same message more than once to the same address is almost certainly not an intended activity.¶
Email handling information is a matter of modifying the email infrastructure and possibly creating privacy concerns. A header field such as this is not automatically assured of adoption or use. Therefore it is being published as an Experiment, looking for constituency and for operational utility.¶
This document is produced through the Independent RFC stream and was not subject to the IETF's approval process.¶
Unless otherwise indicated, basic architecture and terminology used in this document are taken from:¶
and syntax is specified with:¶
Normative language, per [RFC8174]:¶
RFC EDITOR: Remove before publication:¶
This document defines the "Delivered-To" header field, for annotating am email delivery event and the individual mailbox to which delivery has been effected.¶
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 mailbox. The header field contains the individual mailbox used to determine the immediate location for that transition.¶
The syntax of the header field is:¶
"Delivered-To:" FWS Mailbox CRLF
; Mailbox is from [SMTP]¶
The Delivered-To header field can indicate a sequence of deliveries, as demonstrated by this example, which has a message traveling through a mailing list, on through an alias, and then reaching final delivery:¶
List @ org.example¶
Delivered-To: list@org.example Received: by submit.org.example with SMTP id i17so17480689ljn.1 for <list@org.example>; Mon, 25 Jan 2021 15:29:19 -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) Delivered-To: list@org.example Received: by submit.org.example with SMTP id i17so17480689ljn.1 for <list@org.example>; Mon, 25 Jan 2021 15:29:19 -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) Delivered-To: list@org.example Received: by submit.org.example with SMTP id i17so17480689ljn.1 for <list@org.example>; Mon, 25 Jan 2021 15:29:19 -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, their presence in a message discloses handling information and, possibly, personal information.¶
In some email implementations, a message that is for multiple (local) recipients has a single stored version. Supporting Delivered-To, while maintaining recipient privacy, creates a challenge. However, exposing different mailboxes to other recipients MUST NOT occur.¶
An issue specific to this mechanism is disclosure of a sequence of mailboxes, if a message goes through a series of recipient envelope mailbox modifications. The document calls for each of these mailboxes to be recorded in separate Delivered-To fields. This does not disclose mailboxes of other recipients, but it does disclose a mailbox-transformation handling path for the recipient.¶
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.¶
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 Adrian Farrel, Ned Freed, Barry Leiba, John Levine, Brandon Long, Michael Peddemors, Phil Pennock, Alessandro Vesely.¶