The Delivered-To Message Header Field

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/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 17 February 2022.¶

Copyright Notice

Copyright (c) 2021 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

Table of Contents

1. Introduction

The Delivered-To message header 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 to detect and break delivery loops.¶

2. Header Syntax

The syntax of the Delivered-To header is similar to other mail message headers. In the ABNF below, addr-spec is imported from [RFC5322].¶

delivered-to = "Delivered-To:" addr-spec CRLF

The contents of the header 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, so the header contents do not collide with headers created by other MDAs.¶

3. Related Headers

Some MDAs add a different header that records the actual RCPT TO address in an SMTP or submission session that handled the message. This header is typically called Envelope-To or X-Original-To, but varies from one MDA to another.¶

4. IANA Considerations

IANA is requested to add the following entry to the Permanent Message Header Field Names registry:¶

5. Security Considerations

Depending on the way that an MDA creates the Delivered-To header, it may be possible to guess internal details of the delivery process from the contents of the header. 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 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.¶

Authors' Addresses