| Internet-Draft | react | February 2021 |
| Crocker | Expires 20 August 2021 | [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 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.¶
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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. One use can be for detecting a delivery sequence loop.¶
Unless otherwise indicated, basic architecture and terminology used in this specification are taken from:¶
and syntax is specified with:¶
Normative language, per [RFC8174]:¶
RFC EDITOR: Remove before publication:¶
This specification defines the "Delivered-To" header field, for annotating a delivery event and the individual address to which delivery has been 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 some other information, each additional Delivered-To: header field MUST be placed at the current 'top' of the message -- as the first header field, in a fashion similar to some fields specified in [SMTP], such as in Section 4.1.1.4. In addition, and as with other fields placed at the current top, the Delivered-To header field MUST NOT be reordered, with respect to other Delivered-to fields AND those other fields.¶
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 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
; 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 @ example.org¶
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>¶
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¶
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¶
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 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.¶
Registration of the "Delivered-To" header field is requested, per [RFC3864]:¶
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, Alessandro Vesely.¶