Internet-Draft | deliveredto | February 2022 |
Dukhovni & Levine | Expires 10 August 2022 | [Page] |
- Workgroup:
- Network Working Group
- Internet-Draft:
- draft-duklev-deliveredto-01
- Published:
- Intended Status:
- Informational
- Expires:
The Delivered-To Message Header Field
Abstract
This document describes the existing usage of the Delivered-To header field in e-mail messages.¶
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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 10 August 2022.¶
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. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
1. Introduction
The Delivered-To message header field was introduced by the [qmail] mail package in 1998, and adopted shortly afterward by [Postfix] and [Courier]. Mail Delivery Agents (MDA) [RFC5598] use the header field to detect and break delivery loops.¶
2. Header Field Syntax
The syntax of the Delivered-To header field is similar to other mail message header fields. In the ABNF below, addr-spec is imported from [RFC5322].¶
delivered-to = "Delivered-To:" addr-spec CRLF¶
The contents of the header field is an opaque string that is an MDA-specific representation of the mailbox to which a message was delivered. The string need not be, and often is not, an address to which mail can be sent using SMTP.¶
The domain part of the addr-spec is typically a mail domain managed by the MDA adding the header field, so the header field contents do not collide with header fields created by other MDAs.¶
2.1. Loop Breaking
Some MDAs use the Delivered-To header field to break delivery loops using the following method.¶
When an MDA is about to deliver a message, it creates a Delivered-To header field that represents the target of the delivery, and then scans the existing header fields in the message to see if an identical Delivered-To header field is already present. If so, the message is in a loop, and the delivery fails. If not, the MDA prepends the header field to the message and proceeds with the delivery.¶
Delivered-To header fields are typically prepended to the message, similarly to the way trace header fields are prepended, but the loop breaking algorithm does not depend on the order of the header fields, only whether an identical header field is already present.¶
4. IANA Considerations
IANA is requested to add the following entry to the Permanent Message Header Field Names registry:¶
Header Field Name | Template | Protocol | Status | Reference |
---|---|---|---|---|
Delivered-To | (blank) | informational | [this document] |
5. Security Considerations
Depending on the way that an MDA creates the Delivered-To header field, it may be possible to guess internal details of the delivery process from the contents of the header field. To avoid this, some MDAs may obscure the Delivered-To contents by hashing or otherwise transforming the part of contents to the left of the @-sign to make it harder to reverse engineer.¶
Malicious senders have occasionally sent messages with a Delivered-To header field that deliberately matches the one to be added by an MDA, to provoke a bounce from that MDA to the envelope sender of the message, causing what is known as "blowback spam." Mitigations are the same as for any undeliverable mail that may have a forged envelope sender address.¶
6. Acknowledgments
We thank Sam Varshavchik for his reviews and useful suggestions.¶
7. Informative References
- [Courier]
- Varshavchik, S., "Courier Mail Server", , <https://courier-mta.org>.
- [Postfix]
- Venema, W., "Postfix", , <http://www.postfix.org>.
- [RFC5322]
- Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, , <https://www.rfc-editor.org/info/rfc5322>.
- [RFC5598]
- Crocker, D., "Internet Mail Architecture", RFC 5598, DOI 10.17487/RFC5598, , <https://www.rfc-editor.org/info/rfc5598>.
- [qmail]
- Bernstein, D.J., "qmail", , <https://cr.yp.to/qmail.html>.