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Versions: 00 01

Network Working Group                                           S. Blank
Internet-Draft                                              P. Goldstein
Intended status: Experimental                                   Valimail
Expires: 1 February 2021                                   T. Loder, Ed.
                                                    Skye Logicworks, LLC
                                                            T. Zink, Ed.
                                               Zink Magical Contraptions
                                                        M. Bradshaw, Ed.
                                                                Fastmail
                                                            31 July 2020


           Brand Indicators for Message Identification (BIMI)
                        draft-blank-ietf-bimi-01

Abstract

   Brand Indicators for Message Identification (BIMI) permits Domain
   Owners to coordinate with Mail User Agents (MUAs) to display brand-
   specific Indicators next to properly authenticated messages.  There
   are two aspects of BIMI coordination: a scalable mechanism for Domain
   Owners to publish their desired Indicators, and a mechanism for Mail
   Transfer Agents (MTAs) to verify the authenticity of the Indicator.
   This document specifies how Domain Owners communicate their desired
   Indicators through the BIMI Assertion Record in DNS and how that
   record is to be interpreted by MTAs and MUAs.  MUAs and mail-
   receiving organizations are free to define their own policies for
   making use of BIMI data and for Indicator display as they see fit.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 1 February 2021.

Copyright Notice

   Copyright (c) 2020 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 (http://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
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Overview
   3.  Requirements
       3.1.  High-Level Goals
       3.2.  Security
       3.3.  Out of Scope
   4.  Terminology and Definitions
       4.1.  BIMI Assertion
       4.2.  Indicator
       4.3.  Mark Verifying Authority (MVA)
       4.4.  BIMI Evidence Document
       4.5.  Verified Mark Certificate (VMC)
       4.6.  Protocol Client
       4.7.  Verifying Protocol Client
   5.  BIMI DNS Records
       5.1.  MUA Obligations
       5.2.  Assertion Record
             5.2.1.  Declination to Publish
             5.2.2.  Supported Image Formats for l= tag
       5.3.  Selectors
   6.  BIMI Header Fields
       6.1.  BIMI-Selector Header
       6.2.  BIMI-Location Header
       6.3.  BIMI-Indicator Header
       6.4.  Header Signing
   7.  Domain Owner Actions
       7.1.  Determine and Publish Indicator(s) for Use
       7.2.  Publish Assertion Records
       7.3.  Manage multiple uses of the same Indicator(s) within
             a trust boundary
       7.4.  Set the headers on outgoing email as appropriate
   8.  Receiver Actions
       8.1.  Authentication Requirements
       8.2.  Assertion Record Discovery
       8.3.  Indicator Discovery.
       8.4.  Indicator Discovery With Evidence.
       8.5.  Indicator Discovery Without Evidence.
       8.6.  Indicator Validation
       8.7.  Affix BIMI Status to Authentication Results Header
             Field
       8.8.  Handle Existing BIMI-Location and BIMI-Indicator
             Headers
       8.9.  Construct BIMI-Location URI
       8.10.  Construct BIMI-Indicator header
   9.  Security Considerations
       9.1.  Indirect Mail Flows
       9.2.  Lookalike Domains and Copycat Indicators
       9.3.  Large files and buffer overflows
       9.4.  Slow DNS queries
       9.5.  Unaligned Indicators and asserting domains
       9.6.  Unsigned BIMI-Selector Header
       9.7.  CGI scripts in Indicator payload
       9.8.  Metadata in Indicators
   10.  IANA Considerations
       10.1.  Permanent Header Field Updates
       10.2.  Registry for Supported BIMI Formats
       10.3.  Other IANA needs
   11.  References
       11.1.  Normative References
       11.2.  Informative References
   Appendix A.  Example Selector Discovery (INFORMATIVE)
       A.1.  No BIMI-Selector Header
       A.2.  With BIMI-Selector Header
       A.3.  Without BIMI-Selector Header on a subdomain
       A.4.  With BIMI-Selector Header on a subdomain
       A.5.  Invalid BIMI-Selector Header
   Appendix B.  Example Authentication-Results entry (INFORMATIONAL)
       B.1.  Successful BIMI lookup
       B.2.  No BIMI record
       B.3.  Subdomain has no default record, but organizational
             domain does
       B.4.  Subdomain has no record for selector, but
             organization domain has a default
   Appendix C.  Example BIMI Headers Construction (INFORMATIONAL)
       C.1.  MTA Receives an email
       C.2.  MTA does its authentication checks
       C.3.  MTA performs BIMI Assertion
       C.4.  MTA appends to Authentication-Results
       C.5.  MTA Constructs BIMI-Location and BIMI-Indicator
             headers
       C.6.  The MUA displays the Indicator
       C.7.  Acknowledgements
   Authors' Addresses

1.  Introduction

   This document defines Brand Indicators for Message Identification
   (BIMI), which enables Domain Owners to coordinate with Mail Box
   Providers (MBPs), Mail Transfer Agents (MTAs), and Mail User Agents
   (MUAs) in the display of brand-specific Indicators next to properly
   authenticated messages.

   BIMI is designed to be open and to work at Internet scale.  BIMI is
   intended to drive adoption of email authentication best practices by
   leveraging existing [DMARC] policies, the supporting authentication
   methods [DKIM] and [SPF], and other associated standards such as
   [ARC].

   The approach taken by BIMI is heavily influenced by the approach
   taken in [DKIM], in that BIMI:

   *  has no dependency on the deployment of any new Internet protocols
      or services for Indicator registration or revocation;

   *  makes no attempt to include encryption as part of the mechanism;

   *  is compatible with the existing email infrastructure and
      transparent to the fullest extent possible;

   *  requires minimal new infrastructure;

   *  can be implemented independently of clients in order to reduce
      deployment time;

   *  can be deployed incrementally; and

   *  allows delegation of Indicator hosting to third parties.

   To participate in BIMI, Domain Owners MUST have a strong [DMARC]
   policy (quarantine or reject) on both the Organizational Domain, and
   the RFC5322.From Domain of the message.  Quarantine policies MUST NOT
   have a pct less than pct=100.

   This document defines how Domain Owners specify their desired
   Indicators through the BIMI Assertion Record in DNS and how that
   record is to be interpreted by MTAs and MUAs.  This document does not
   cover how domains or Indicators are verified, how MUAs should display
   the Indicators, or how other protocols (i.e.  IMAP, JMAP) can be
   extended to work with BIMI.  Other documents may cover these topics.
   MUAs and Mail Box Providers (MBPs) are free to define their own
   policies for making use of BIMI data and for Indicator display as
   they see fit.

2.  Overview

   The Sender Policy Framework ([SPF]), DomainKeys Identified Mail
   ([DKIM]), Domain-based Message Authentication, Reporting, and
   Conformance ([DMARC]), and Authenticated Received Chain ([ARC])
   provide mechanisms for domain-level authentication of email messages.
   They enable cooperating email senders and receivers to distinguish
   messages that are authorized to use the domain name from those that
   are not.  BIMI relies on these authentication protocols, but is not a
   new authentication protocol itself.

   MUAs are increasingly incorporating graphical Indicators to indicate
   the identity of the sender of a message.  While a discussion of the
   merits of doing this is beyond the scope of this document, at present
   there are no open standards for publishing and aiding discovery of
   preferred Indicators or for tying display of them to authentic
   messages only.

   Because of the desire to have brand-specific Indicators available,
   some mail-receiving organizations have developed closed systems for
   obtaining and displaying Brand Indicators for select domains.  While
   this has enabled these mail-receiving organizations to display brand
   Indicators for a limited subset of messages, this closed approach has
   a number of downsides:

   *  It puts a significant burden on each mail-receiving organization,
      because they must identify and manage a large database of Brand
      Indicators.

   *  Scalability is challenging for closed systems that attempt to
      capture and maintain complete sets of data across the whole of the
      Internet.

   *  A lack of uniformity across different mail-receiving organizations
      - each organization will have its own Indicator set, which may or
      may not agree with those maintained by other organizations for any
      given domain.

   *  Domain Owners have limited ability to influence the Brand
      Indicator for the domain(s) they own, and any ability they do have
      is likely to be dependent upon direct coordination with each of
      many mail-receiving organizations.

   *  Many Domain Owners have no ability to participate whatsoever as
      they do not have the appropriate relationships to coordinate with
      mail-receiving organizations.

   *  MUAs that are not associated with a particular mail-receiving
      organization are likely to be disadvantaged, because they are
      unlikely to receive Indicators in a standardized manner or
      optimized for their user interfaces.

   This shows the need for a standardized mechanism by which Domain
   Owners interested in ensuring that their Indicators are displayed
   correctly and appropriately can publish and distribute Brand
   Indicators for use by any participating MUA.

   BIMI removes the substantial burden of curating and maintaining an
   Indicator database from MUAs and MBPs, and allows each domain owner
   to manage their own Indicators.  As an additional benefit, mail-
   originating organizations are incentivized to authenticate their
   email as doing so will enable them to influence how email and
   Indicators from the organization are displayed.

   The structure of BIMI is as follows:

   *  Domain Owners: Publish their preferred Brand Indicators via the
      [DNS].

   *  Senders: Ensure mail is properly authenticated, and has a
      sufficiently strict [DMARC] policy.

   *  MTA:

      -  Confirm authenticity of the message using [DMARC] and whatever
         other authentication mechanisms they wish to apply.

      -  Check for a corresponding BIMI record, obtaining references to
         the Indicator media and optional substantiation of Indicator
         ownership rights

      -  If both the message is authentic and the Indicator is deemed
         acceptable, the receiver adds a header to the message which can
         be used by the MUA to obtain the Domain Owner's preferred Brand
         Indicator.

   *  MUA: retrieves and displays the Brand Indicator as appropriate
      based on its policy and user interface.

   The purpose of this structure is to reduce operational complexity at
   each step.  It is also to consolidate validation and Indicator
   selection operations into the MTA, so that Domain Owners need only
   publish a few simple records and MUAs only need simple display logic.

   It is expected that MBPs implementing BIMI will do so in both their
   MTAs and MUAs.

3.  Requirements

   Specification of BIMI in this document is guided by the following
   high-level goals, security dependencies, detailed requirements, and
   items that are documented as out of scope.

   An overview of the security challenges and design decisions is
   documented at [BIMI-OVERVIEW].

3.1.  High-Level Goals

   BIMI has the following high-level goals:

   *  Allow Domain Owners to suggest appropriate Indicators for display
      with authenticated messages originating from their domains.

   *  Enable the authors of MUAs to display meaningful Indicators
      associated with the Domain Owner to recipients of authenticated
      email.

   *  Provide mechanisms to prevent attempts by malicious Domain Owners
      to fraudulently represent messages from their domains as
      originating with other entities.

   *  Work at Internet Scale.

   *  Encourage the adoption of Email Authentication Best Practices.

3.2.  Security

   Brand Indicators are a potential vector for abuse.  BIMI creates a
   relationship between sending organization and Mail Receiver so that
   the receiver can display appropriately designated Indicators if the
   sending domain is verified and has meaningful reputation with the
   receiver.  Without verification and reputation, there is no way to
   prevent a bad actor exxample.com from using example.com's Brand
   Indicators and behaving maliciously.  This document does not cover
   the different verification and reputation mechanisms available, but
   BIMI relies upon them to be in deployed in order to control abuse.

3.3.  Out of Scope

   Several topics and issues are specifically out of scope for the
   initial version of this work.  These include the following:

   *  Publishing policy other than via the DNS.

   *  Specific requirements for Indicator display on MUAs.

   *  The explicit mechanisms used by Verifying Protocol Clients - this
      will be deferred to a later document.

4.  Terminology and Definitions

   This section defines terms used in the rest of the document.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [KEYWORDS].

   Readers are encouraged to be familiar with the contents of [EMAIL-
   ARCH].  In particular, that document defines various roles in the
   messaging infrastructure that can appear the same or separate in
   various contexts.  For example, a Domain Owner could, via the
   messaging mechanisms on which BIMI is based, delegate reponsibility
   for providing preferred Brand Indicators to a third party with
   another role.  This document does not address the distinctions among
   such roles; the reader is encouraged to become familiar with that
   material before continuing.

   Syntax descriptions use Augmented BNF [ABNF].

   "Author Domain", "Domain Owner", "Organizational Domain", and "Mail
   Receiver" are imported from [DMARC] Section 3.

4.1.  BIMI Assertion

   The mechanism through which a Protocol Client verifies the BIMI
   Assertion Record and constructs the URI(s) to the requested
   Indicator(s) to be placed in the BIMI-Location header.

4.2.  Indicator

   The icon, logo, image, mark, or other graphical representation of the
   brand.  The Indicator is defined in a common image format with
   restrictions detailed in the Assertion Record definition
   (Section 5.2).

4.3.  Mark Verifying Authority (MVA)

   An entity or organization that can provide evidence of verification
   of Indicators asserted by a Domain Owner to Verifying Protocol
   Clients.  The MVA may choose to uphold and confirm the meeting of
   certain Indicator standards (ie. size, trademark, content, etc).

4.4.  BIMI Evidence Document

   A document published by a Mark Verifying Authority to assert evicence
   of verification.  These are defined in a separate document.

4.5.  Verified Mark Certificate (VMC)

   A certificate issued by a Certificate Authority in accordance with
   the Verified Mark Certificate Guidelines.  These guidelines are
   defined in a separate document.

   A Verified Mark Certificate is one example of a BIMI Evidence
   Document.

4.6.  Protocol Client

   An entity designed to obtain and correctly interpret the records
   defined in this specification for the purpose of discovering and
   fetching published Indicators.

4.7.  Verifying Protocol Client

   A Protocol Client that uses optional capabilities to obtain and
   evaluate evidence concerning the Domain Owner's rights to use the
   published Indicators.

5.  BIMI DNS Records

   Domain owners publish BIMI policies by adding BIMI Assertion Records
   in the DNS as TXT records.

   Published policies are interpreted and applied by Protocol Clients.
   A Domain Owner signals intended BIMI participation for one or more of
   its domains by publishing an Assertion Record in a subdomain under
   it.  In doing so, Domain Owners make specific requests of MUAs
   regarding the preferred set of Indicators to be displayed with
   messages that are confirmed to be authorized to appear from the
   Domain Owner's domain.

   The use of BIMI is opt-in.  Receivers default to performing no BIMI-
   specific message handling until they choose to do so, and then only
   if a BIMI record for the sender's domain is found.

   BIMI's use of the DNS is driven in part by BIMI's use of domain names
   as the basis of sender identity and message authentication.  Use of
   the DNS as the policy publication service also has the benefit of
   reusing an extremely well-established operations, administration, and
   management infrastructure, rather than creating a new one.

   BIMI's policy payload is intentionally only published via a DNS
   record and not via one or more email headers.  This serves three
   purposes:

   *  There is one and only one mechanism for both simple and complex
      policies to be published.

   *  Operational complexity is reduced.  MTAs only need to check a
      single record in a consistent manner to discover and enforce
      policy.

   *  Indicators SHOULD be verified and cached in advance, so that
      malicious headers cannot be used as an attack vector.

   Per [DNS], a TXT record can comprise several "character-string"
   objects.  BIMI TXT records with multiple strings must be treated in
   an identical manner to SPF Section 3.3 (https://tools.ietf.org/html/
   rfc7208#section-3.3).

5.1.  MUA Obligations

   MUAs implementing the BIMI mechanism SHOULD make a best-effort
   attempt to adhere to the Domain Owner's published BIMI policy.
   However, MUAs have final control over the user interface published to
   their end users, and MAY use alternate Indicators than those
   specified in the BIMI assertion record or no Indicator at all.

5.2.  Assertion Record

   All Domain Owner BIMI preferences are expressed in DNS TXT records
   published in subdomains named "_bimi".  Multiple sets of preferences
   can be associated with a single RFC5322.From domain.  To distinguish
   between these different preferences, BIMI defines and uses selectors.
   Senders declare which selector to use for a given message by
   specifying the selector in an optional BIMI-Selector header
   (Section 6.1).

   For example, the Domain Owner of "example.com" would post BIMI policy
   in a TXT record at "default._bimi.example.com".  Similarly, a Mail
   Receiver wishing to query for BIMI policy regarding mail with an
   RFC5322.From Author Domain of "example.com" and a selector "default"
   (the default) would query the TXT record located at the subdomain of
   "default._bimi.example.com".  The DNS-based BIMI policy record is
   referred to as the "BIMI Assertion Record" or "Assertion Record".

   BIMI Assertion Records follow the extensible "tag-value" syntax for
   DNS-based key records as defined in [DKIM].

   Assertion Records are defined precisely.  Mail receivers MUST NOT
   attempt to fix syntactical or capitalization errors.  If a required
   tag is missing, or its value not well-formed, it is an error.

   This section creates a registry for known BIMI tags and registers the
   initial set defined in this document.  Only tags defined in this
   document or in later extensions, and thus added to the registry, are
   to be processed; unknown tags MUST be ignored.

   The following tags are introduced as the initial valid BIMI tags:

   v= Version (plain-text; REQUIRED).  Identifies the record retrieved
   as a BIMI record.  It MUST have the value of "BIMI1" for
   implementations compliant with this version of BIMI.  The value of
   this tag MUST match precisely; if it does not match or it is absent,
   the entire retrieved record MUST be ignored.  It MUST be the first
   tag in the list.

   ABNF:

   bimi-version = %x76 *WSP "=" *WSP %x42.49.4d.49 1DIGIT

   a= Authority Evidence Location (plain-text; URI; OPTIONAL).  If
   present, this tag MUST have an empty value or its value MUST be a
   single URI.  An empty value for the tag is interpreted to mean the
   Domain Owner does not wish to publish or does not have authority
   evidence to disclose.  The URI, if present, MUST contain a fully
   qualified domain name (FQDN) and MUST specify HTTPS as the URI scheme
   ("https").  The URI SHOULD specify the location of a publicly
   retrievable BIMI Evidence Document.  The format for evidence
   documents is defined in a separate document.

   If the a= tag is not present, it is assumed to have an empty value.

   ABNF:

   bimi-evidence-location = %x61 *WSP "=" bimi-uri

   bimi-uri = \[FWS\] URI \[FWS\]

   ; "URI" is imported from [URI]
   ; HTTPS only
   ; commas within a URI (ASCII ; 0x2C) MUST be encoded

   l= location (URI; REQUIRED).  The value of this tag is either empty
   indicating declination to publish, or a single URI representing the
   location of a Brand Indicator file.  The only supported transport is
   HTTPS.

   ABNF:

   bimi-location = %x6c *WSP "=" bimi-uri

   Therefore, the formal definition of the BIMI Assertion Record, using
   [ABNF], is as follows:

   bimi-sep = *WSP %x3b *WSP

   bimi-record = bimi-version (bimi-sep bimi-location) (bimi-sep bimi-evidence-location) \[bimi-sep\]

   ; components other than bimi-version may appear in any order

5.2.1.  Declination to Publish

   If both the "l=" and "a=" tags are empty, it is an explicit refusal
   to participate in BIMI.  This is distinct from not publishing a BIMI
   record.  For example, an empty BIMI record enables a Domain Owner to
   decline BIMI participation for a subdomain when its organizational
   domain has default Indicators available.  Furthermore, messages sent
   using a selector that has declined to publish will not show an
   Indicator while messages with other selectors would display normally.

   An explicit declination to publish looks like:

   v=BIMI1; l=; a=;

5.2.2.  Supported Image Formats for l= tag

   Any format in the BIMI-formats IANA registry are acceptable targets
   for the l= tag.  If an l= tag URI ends with any other image format
   suffix, or if the document retrievable from the location(s) in the l=
   tag are of any other format, the evaluation of the record MUST be
   treated as a permanent error.

   As of the publishing of this document, only SVG and SVGZ, as defined
   in RFC6170 section 5.2 (https://tools.ietf.org/html/rfc6170#section-
   5.2) is acceptable in the l= tag.  Further restrictions apply to the
   SVG; these are documented elsewhere.

5.3.  Selectors

   To support publishing and display of more than one distinct Brand
   Indicator per domain, the brand Indicator namespace is subdivided for
   publishing of multiple Assertion Records using "selectors".
   Selectors allow the Domain Owner to choose the brand Indicator, for
   example, by type of recipient, by message source, or by other
   considerations like seasonal branding.  BIMI selectors are modeled
   after DKIM selectors (https://tools.ietf.org/html/rfc6376#section-
   3.1).

   The selector "default" is the default Assertion Record.  Domain
   Owners can specify which other selector to use on a per-message basis
   by utilizing the BIMI-Selector Header (Section 6.1).

   Periods are allowed in selectors and are component separators.  When
   BIMI Assertion Records are retrieved from the DNS, periods in
   selectors define DNS label boundaries in a manner similar to the
   conventional use in domain names.  In a DNS implementation, this can
   be used to allow delegation of a portion of the selector namespace.

   ABNF:

   selector = sub-domain *( "." sub-domain )

   ; from [SMTP] Domain,

   ; excluding address-literal

   The number of selectors for each domain is determined by the Domain
   Owner.  Many Domain Owners will be satisfied with just one selector,
   whereas organizations with more complex branding requirements can
   choose to manage disparate selectors.  BIMI sets no maximum limit on
   the number of selectors.

6.  BIMI Header Fields

   Once BIMI policies are published in DNS via Assertion Records, Domain
   Owners can provide additional guidance to Mail Receivers, and Mail
   Receivers to their MUAs through the use of BIMI header fields.

   BIMI header fields are case insensitive.  If a required tag is
   missing, it is an error.

6.1.  BIMI-Selector Header

   BIMI DNS records are placed in <selector>._bimi.<domain>, and by
   default they are placed in default._bimi.<domain>.  That is, for
   example.com, the default Assertion Record is located in the DNS at
   default._bimi.example.com.  However, a Domain Owner may override the
   use of the default selector and specify the use of an alternative
   using the RFC5322-compliant header 'BIMI-Selector'.  The BIMI-
   Selector header consists of key value pairs:

   v= Version (plain-text; REQUIRED).  The version of BIMI.  It MUST
   have the value of "BIMI1" for implementations compliant with this
   version of BIMI.  The value of this tag MUST match precisely; if it
   does not or it is absent, the entire retrieved record MUST be
   ignored.  It MUST be the first tag in the list.

   ABNF:

   bimi-header-version = "v" *WSP "=" *WSP "BIMI" 1DIGIT

   s= Selector (plain-text; REQUIRED).  The location of the BIMI DNS
   record, when combined with the RFC5322.From domain.

   ABNF:

   bimi-selector = "s" *WSP "=" *WSP selector

   And the formal definition of the BIMI Selector Header, using ABNF, is
   as follows:

   bimi-selector-header = bimi-header-version bimi-sep bimi-selector \[bimi-sep\]

6.2.  BIMI-Location Header

   BIMI-Location is the header a Mail Receiver inserts that tells the
   MUA where to get the BIMI Indicator from.

   The syntax of the header is as follows:

   v= Version (plain-text; REQUIRED).  The version of BIMI.  It MUST
   have the value of "BIMI1" for implementations compliant with this
   version of BIMI.  The value of this tag MUST match precisely; if it
   does not or it is absent, the entire header MUST be ignored.  It MUST
   be the first tag in the list.

   The ABNF for bimi-header-version is imported exactly from the [BIMI Selector Header](#bimi-selector).

   l: location of the BIMI Indicator (URI; OPTIONAL if a bimi-evidence-
   location-header-uri is specified, otherwise REQUIRED.).  Inserted by
   the MTA after performing the required checks and obtaining the
   applicable domain's published Assertion Record.  The value of this
   tag is a URI representing the location of the Brand Indicator file.
   HTTPS is the only supported transport.

   ABNF:

   bimi-location-header-uri = "l" *WSP "=" bimi-uri

   a: location of the BIMI Evidence Document (URI; REQUIRED if the BIMI
   Evidence Document was verified).  Inserted by the MTA after
   performing the required checks and obtaining the applicable domain's
   published Assertion Record.  The value of this tag is a URI
   representing the location of the BIMI Evidence Document.  HTTPS is
   the only supported transport.

   ABNF:

   bimi-evidenced-location-header-uri = "a" *WSP "=" bimi-uri

   And the formal definition of the BIMI Location Header, using ABNF, is
   as follows:

   bimi-location-header-location-only = bimi-location-header-uri

   bimi-location-header-evidence-only = bimi-evidence-location-header-uri

   bimi-location-header-both = bimi-location-header-uri bimi-evidence-location-header-uri

   bimi-location-options = bimi-location-header-location-only / bimi-location-header-evidence-only / bimi-location-header-both

   bimi-location-header = bimi-header-version bimi-sep bimi-location-options \[bimi-sep\]

6.3.  BIMI-Indicator Header

   BIMI-Indicator is the header a Mail Receiver inserts to pass a
   verified Indicator to the MUA.

   The header contains the SVG of the Indicator encoded as base64, and
   is inserted by the MTA after performing the required checks and
   obtaining the applicable domain's published Assertion Record.  The
   contents of this tag MUST match the SVG Indicator content retrieved
   from the URI specified in the BIMI-Location header.  If he Indicator
   was supplied as a gzipped SVGZ file then the MTA MUST uncompress the
   file before base64 encoding.

   base64string    =  ALPHADIGITPS *([FWS] ALPHADIGITPS)
                      [ [FWS] "=" [ [FWS] "=" ] ]

   And the formal definition of the BIMI Indicator Header, using ABNF,
   is as follows:

   bimi-indicator-header = bimi-sep base64string \[bimi-sep\]

6.4.  Header Signing

   If present, the BIMI-Selector header SHOULD be included in the DMARC-
   aligned DKIM signature used to confirm authenticity of the message.
   If it is not included in the DMARC-compliant DKIM signature, the
   header SHOULD be ignored.

   Receivers MAY choose to apply additional methods to validate the
   BIMI-Selector header, for example by evaluating a trusted [ARC]
   chain.  In this case the Receiver MAY choose to treat the message as
   if the BIMI-Selector header was signed.

   The BIMI-Location and BIMI-Indicator headers MUST NOT be DKIM signed.
   This header is untrusted by definition, and is only for use between
   an MTA and its MUAs, after DKIM has been validated by the MTA.
   Therefore, signing this header is meaningless, and any messages with
   it signed are either coming from malicious or misconfigured third
   parties.

7.  Domain Owner Actions

   This section includes a walk through of the actions a Domain Owner
   takes when setting up Assertion Records and sending email messages.

7.1.  Determine and Publish Indicator(s) for Use

   Domain Owners should consider which Indicator file formats to choose
   when setting up their BIMI Assertion Records.  For a Sender, BIMI
   provides control over which Indicators are eligible and can be chosen
   for display, but not the ultimate manner in which the MUA will
   display the Indicator.

7.2.  Publish Assertion Records

   For each set of Indicators and domains, publish the appropriate
   Assertion Record as either "default" or a named selector as a DNS TXT
   record within the appropriate "_bimi" namespace.

7.3.  Manage multiple uses of the same Indicator(s) within a trust boundary

   For Domain Owners with multiple domains that wish to share the same
   set of Indicators within a trust boundary and only manage those
   Indicators from a single DNS location, it is RECOMMENDED to use DNS
   CNAMEs.

   Using a CNAME here is functionally similar to the SPF redirect
   modifier.  Since BIMI does not require l= tags to be aligned to the
   Author Domain, CNAMEs present a cleaner solution than extending the
   protocol.

7.4.  Set the headers on outgoing email as appropriate

   Once a default Assertion Record has been published for an Author
   Domain, all emails from this domain should display the appropriate
   Indicator in participating MUAs.

   If a non-default Indicator is desired, the BIMI-Selector header
   should be set appropriately.  If for some reason this selector cannot
   be accessed by the Protocol Client, the fallback is the default
   Assertion Record on the Organization domain.

   The BIMI-Location header MUST NOT be set by email senders, and
   Protocol Clients MUST ignore it.

8.  Receiver Actions

   This section includes a walk through of the actions a Protocol Client
   takes when evaluating an email message for BIMI Assertion.

8.1.  Authentication Requirements

   Before applying BIMI processing for a message, a receiver MUST verify
   that the message passed the following BIMI authentication
   requirements:

   1.  If more than 1 RFC5322.From header is present in the message, or
       any RFC5322.From header contains more than 1 email address then
       BIMI processing MUST NOT be performed for this message.

   2.  Start with the DNS domain found in the RFC5322.From header in the
       message.  Define this DNS domain as the Author Domain.

   3.  Find the Organizational Domain for the Author Domain.  Define
       this DNS domain as the Author Organizational Domain.  If the
       Author Domain is an Organizational Domain then this will be
       identical to the Author Domain.

   4.  Evaluate the [DMARC] result for the Author Domain.  Define the
       result as the BIMI DMARC Result.

   5.  If the BIMI DMARC result is not 'pass', then the receiver MAY
       choose to apply additional authentication methods, for example by
       evaluating a trusted [ARC] chain, a list of trusted forwarders,
       or by applying a local policy.  In this case the Receiver MAY
       choose to treat the message as if the BIMI DMARC Result was
       'pass'.

   6.  If the [DMARC] result for the Author Domain is not 'pass', and
       the message could not be authenticated by any additional
       authentication method, then BIMI processing MUST NOT be performed
       for this message.

   7.  If the [DMARC] policy for the Author Domain or Author
       Organizational Domain is p=none then BIMI processing MUST NOT be
       performed for this message.

   8.  IF the [DMARC] record for the Author Domain or Author
       Organizational Domain includes a subdomain policy, and that
       subdomain policy is sp=none then BIMI processing MUST NOT be
       performed for this message.

   9.  If the [DMARC] policy for the Author Domain or Author
       Organizational Domain is p=quarantine, and the [DMARC] record
       defines a percentage tag, then that tag MUST be pct=100,
       otherwise BIMI processing MUST NOT be performed for this message.

8.2.  Assertion Record Discovery

   Through the BIMI Assertion Record (Section 5.2), Domain Owners use
   DNS TXT records to advertise their preferences.  Preference discovery
   is accomplished via a method similar to the method used for [DMARC]
   records.  This method, and the important differences between BIMI and
   [DMARC] mechanisms, are discussed below.

   Assertion Record Discovery MUST NOT be attempted if the message
   authentication fails per Receiver policy.

   To balance the conflicting requirements of supporting wildcarding,
   allowing subdomain policy overrides, and limiting DNS query load,
   Protocol Clients MUST employ the following lookup scheme for the
   appropriate BIMI record for the message:

   1.  Start with the DNS domain found in the RFC5322.From header in the
       message.  Define this DNS domain as the Author Domain.

   2.  If the message for which the Indicator is being determined
       specifies a selector value in the BIMI Selector Header
       (Section 6.1), use this value for the selector.  Otherwise the
       value 'default' MUST be used for the selector.

   3.  Clients MUST query the DNS for a BIMI TXT record at the DNS
       domain constructed by concatenating the selector, the string
       '_bimi', and the Author Domain.  A possibly empty set of records
       is returned.

   4.  Records that do not start with a "v=" tag that identifies the
       current version of BIMI MUST be discarded.

   5.  If the set is now empty, the Client MUST query the DNS for a BIMI
       TXT record at the DNS domain constructed by concatenating the
       selector 'default', the string '_bimi', and the Organizational
       Domain (as defined in [DMARC]) corresponding to the Author
       Domain.  A custom selector that does not exist falls back to
       default._bimi.<organizationalDomain>, and NOT
       <selector>._bimi.<organizationalDomain>.  A possibly empty set of
       records is returned.

   6.  Records that do not start with a "v=" tag that identifies the
       current version of BIMI MUST be discarded.

   7.  If the remaining set contains multiple records or no records,
       Assertion Record Discovery terminates and BIMI processing MUST
       NOT be performed for this message.

   8.  If the remaining set contains only a single record, this record
       is used for BIMI Assertion.

8.3.  Indicator Discovery.

   1.  If the retrieved Assertion Record does not include a valid bimi-
       location in the l= tag, then Indicator Discovery has failed, and
       the Indicator MUST NOT be displayed.  The bimi-location entry
       MUST be a URI with a HTTPS transport.

   2.  If the retrieved Assertion Record includes a bimi-evidence-
       location entry in the a= tag, and the receiver supports BIMI
       Evidence Document validation, then proceed to the Indicator
       Discovery With Evidence (Section 8.4) step.

   3.  If the receiver does not support BIMI Evidence Document
       validation, or the retrieved Assertion Record does not include a
       bimi-evidence-location entry, then proceed to the Indicator
       Discovery Without Evidence (Section 8.5) step.

8.4.  Indicator Discovery With Evidence.

   Individual types of BIMI Evidence Document MAY specify extra
   discovery and validation steps.  These will be defined in separate
   documents.

8.5.  Indicator Discovery Without Evidence.

   If an Assertion Record is found, and has an empty or missing bimi-
   evidence-location entry then no evidence has is presented, and the
   Indicator MUST be retrieved from the URI specified in the bimi-
   location entry using the following algorithm:

   1.  Retrieve the SVG Indicator from the URI specified in the l= tag.
       This MUST be a URI with a HTTPS transport.

   2.  If the TLS server identity certificate presented during the TLS
       session setup does not chain-up to a root certificate the Client
       trusts then Indicator validation has failed and the Indicator
       MUST NOT be displayed.

   3.  Proceed to the Indicator Validation (Section 8.6) step.

8.6.  Indicator Validation

   1.  Check the file size of the retrieved Indicator against
       recommended maximum sizes as defined in this document, and in the
       BIMI SVG document.  A receiver MAY choose to implement their own
       file size restrictions.  If the Indicator is larger than the
       maximum size the the receiver MAY choose not to display the
       Indicator.  A receiver MAY choose to implement the size limit as
       a retrieval limit rather than retrieving the entire document and
       then checking the size.

   2.  If the SVG Indicator is missing, or is not a valid SVG or SVGZ
       document then validation has failed and the Indicator MUST NOT be
       displayed.

   3.  Check the retrieved Indicator against the SVG validation steps
       specified in this document, and in the BIMI SVG document.  (Note
       to WG, do we want to specify the SVG_1.2_PS profile here or leave
       that to the other document?)

   4.  If Indicator verification has passed, and the Indicator is from a
       trusted source, then the Indicator MAY be displayed per receiver
       policy.

   (Note to WG, add link to BIMI SVG document)

8.7.  Affix BIMI Status to Authentication Results Header Field

   Upon completion of Assertion Record Discovery, Indicator Discovery,
   and Indicator Validation, an MTA SHOULD affix the result in the
   Authentication-Results header using the following syntax, with the
   following key=value pairs:

   bimi: Result of the bimi lookup (plain-text; REQUIRED).  Range of
   values are 'pass' (BIMI successfully validated), 'none' (no BIMI
   record present), 'fail' (syntax error in the BIMI record, failure in
   Discovery or Validation steps, or some other error), 'temperror' (DNS
   lookup problem), or 'skipped' (BIMI check was not performed, possibly
   because the message did not comply with the minimum requirements such
   as passing DMARC, or the MTA does not trust the sending domain).  The
   MTA MAY put comments in parentheses after bimi result, e.g.,
   "bimi=fail (Invalid SVG)", "bimi=skipped (sender not trusted)" or
   "bimi=skipped (message failed DMARC)".

   header.d: Domain of the BIMI Assertion Record which was evaluated
   (plain-text; REQUIRED if bimi=pass).  For example, this will be the
   organizational domain if the BIMI lookup used the fallback record,
   otherwise it will be the RFC5322.From domain.

   header.selector: Selector of the BIMI Assertion Record which was
   evaluated (plain-text; REQUIRED if bimi=pass).  For example, if a
   BIMI-Selector Header was present and used to discover a BIMI
   Assertion Record then this will be the Selector used, otherwise this
   will be 'default'.

   policy.authority: Authority verification status of the Brand
   Identifier (plain-text; REQUIRED if the BIMI Evidence Document was
   checked).  If the Authority Evidence presented in the BIMI Assertion
   Record was checked and found to be valid then this MUST be set to
   pass.  If the validation failed then this MUST be set to fail.  If no
   Authority Evidence was presented, or the MTA did not check the
   Authority Evidence then this SHOULD be set to none.

   policy.authority-uri: The URI of the BIMI Evidence Document checked,
   as found in the a= tag of the BIMI Assertion Record (plain-text;
   OPTIONAL).

8.8.  Handle Existing BIMI-Location and BIMI-Indicator Headers

   Regardless of success of the BIMI lookup, if a BIMI-Location or a
   BIMI-Indicator header is already present in a message it MUST be
   either removed or renamed.  This is because the MTA performing BIMI-
   related processing immediately prior to a Mail Delivery Agent (or
   within the same administrative realm) is the only entity allowed to
   specify the BIMI-Location or BIMI-Indicator headers (e.g. not the
   sending MTA, and not an intermediate MTA).  Allowing one or more
   existing headers through to a MUA is a security risk.

   If the original email message had a DKIM signature, it has already
   been evaluated.  Removing the BIMI-Location header at this point
   should not invalidate the signature since it should not be included
   within it per this spec.

8.9.  Construct BIMI-Location URI

   This header MUST NOT be added if Discovery or Validation steps
   failed.

   The URI used to retrieve the validated SVG Indicator.  If the
   receiver extracted the Indicator from the BIMI Evidence Document then
   this SHOULD be the bimi-evidence-location added with a a= tag,
   otherwise it SHOULD be the bimi-location added with a l= tag.  If
   both a= and l= tags are included then the MTA MUST perform checks to
   ensure that the SVG Indicator referenced by the bimi-location is
   identical to the SVG Indicator extracted from the BIMI Evidence
   Document.

8.10.  Construct BIMI-Indicator header

   This header MUST NOT be added if Discovery or Validation steps
   failed.

   Encode the SVG Indicator retrieved and validated during the Indicator
   Discovery and Indicator Validation steps as base64 encoded data.  If
   the Indicator was compressed with gzip when retrieved then the data
   SHOULD NOT be uncompressed before being base64 encoded.

   The MTA MUST fold the header to be within the line length limits of
   [SMTP].

9.  Security Considerations

   The consistent use of Brand Indicators is valuable for Domain Owners,
   Mail Receivers, and End Users.  However, the routine display of brand
   Indicators represents an attractive target for abuse, especially for
   determined malicious actors.  Great care is warranted.  The
   discussion following as an incomplete list of considerations.

9.1.  Indirect Mail Flows

   If a mail store ingests a message from another mail store through
   some other means, the message may or may not have BIMI headers added
   already.  If the receiving store trusts the other mail store, it may
   simply use existing headers.  Or, it may re-evaluate BIMI policy and
   requirements, and create or replace the BIMI-Location header.

9.2.  Lookalike Domains and Copycat Indicators

   Publishing BIMI records is not sufficient for an MTA to signal to the
   MUA to load the BIMI Indicator.  For example, the Domain Owner may
   also need to have a sufficiently strong reputation with the MTA.  The
   receiver may use a manually maintained list of large brands, it may
   import a list from a third party of acceptable domains, or it may
   apply its own reputation heuristics before deciding whether or not to
   load the BIMI Indicator.  BIMI does not specify what MTAs may bring
   to bear as additional factors.

9.3.  Large files and buffer overflows

   The MTA or MUA should perform some basic analysis and avoid loading
   Indicators that are too large or too small.  The Receiver may choose
   to maintain a manual list and do the inspection of its list offline
   so it doesn't have to do it at time-of-scan.

9.4.  Slow DNS queries

   All email Receivers already have to query for DNS records, and all of
   them have built-in timeouts when performing DNS queries.
   Furthermore, the use of caching when loading Indicators can help cut
   down on load time.  Virtually all email clients have some sort of
   image-downloading built-in and make decisions when to load or not
   load Indicators.

9.5.  Unaligned Indicators and asserting domains

   There is no guarantee that a group responsible for managing Brand
   Indicators will have access to put these Indicators directly in any
   specific location of a domain, and requiring that Indicators live on
   the asserted domain is too high a bar.  Additionally, letting a brand
   have Indicator locations outside its domain may be desirable so that
   someone sending legitimate authenticated email on the Domain Owner's
   behalf can manage and set selectors as an authorized third party
   without requiring access to the Domain Owner's DNS or web services.

9.6.  Unsigned BIMI-Selector Header

   If a Domain Owner relies on SPF but not DKIM for email
   authentication, then adding a requirement of DKIM may create too high
   of a bar for that sender.  On the other hand, Receivers doing BIMI
   assertion may factor in the lack of DKIM signing when deciding
   whether to add a BIMI-Location header.

9.7.  CGI scripts in Indicator payload

   MTAs and MVAs should aggressively police Indicators to ensure they
   are the Indicators they claim to be, are within appropriate size
   limits, and pass other sanity checks.  Additionally, MTAs might cache
   good Indicators and serve them directly to their MUAs, which would in
   practice bypass any malicious dynamic payload set to trigger against
   an end user but not an MTA.

9.8.  Metadata in Indicators

   Domain Owners should be careful to strip any metadata out of
   published Indicators that they don't want to expose or which might
   bloat file size.  MTAs and MVAs might wish to inspect and remove such
   data from Indicators before exposing them to end users.

10.  IANA Considerations

   IANA will need to reserve three new entries for the "Permanent
   Message Header Field Names" registry and create a registry for
   support file formats for BIMI.

10.1.  Permanent Header Field Updates

   Header field name: BIMI-Selector

   Applicable protocol: mail

   Status: standard

   Author/Change controller: IETF

   Specification document: This one

   Header field name: BIMI-Location

   Applicable protocol: mail

   Status: standard

   Author/Change controller: IETF

   Specification document: This one

   Header field name: BIMI-Indicator

   Applicable protocol: mail

   Status: standard

   Author/Change controller: IETF

   Specification document: This one

10.2.  Registry for Supported BIMI Formats

   Names of support file types supported by BIMI must be registered by
   IANA.

   New entries are assigned only for values that have been documented in
   a published RFC that has had IETF Review, per [IANA-CONSIDERATIONS].
   Each method must register a name, the file extension, the
   specification that defines it, and a description.

10.3.  Other IANA needs

   NOTE TO WORKING GROUP: The registry for BIMI tags needs to be
   properly set up, as does the registry for validation actions.

11.  References

11.1.  Normative References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [ARC]      Andersen, K., Long, B., Blank, S., and M. Kucherawy, "The
              Authenticated Received Chain (ARC) Protocol", July 2019,
              <http://www.rfc-editor.org/info/rfc8617>.

   [Authentication-Results]
              Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", August 2015,
              <https://tools.ietf.org/html/rfc7601>.

   [DKIM]     Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", September 2011,
              <http://www.rfc-editor.org/info/rfc6376>.

   [DMARC]    Kucherawy, M. and E. Zwicky, "Domain-based Message
              Authentication, Reporting, and Conformance (DMARC)", March
              2015, <http://www.rfc-editor.org/info/rfc7489>.

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", November 1987,
              <http://www.rfc-editor.org/info/rfc1035>.

   [EMAIL-ARCH]
              Crocker, D., "Internet Mail Architecture", July 2009,
              <http://www.rfc-editor.org/info/rfc5598>.

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [Logotype] Santesson, S., Housley, R., and T. Freemani, "Internet
              X.509 Public Key Infrastructure, Logotypes in X.509
              Certificates", February 2004,
              <http://www.rfc-editor.org/info/rfc3709>.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", October
              2008, <http://www.rfc-editor.org/info/rfc5321>.

   [SPF]      Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", April
              2014, <http://www.rfc-editor.org/info/rfc7208>.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

11.2.  Informative References

   [BIMI-OVERVIEW]
              Blank, S., Kumaran, N., and J. Levine, "An Overview of the
              Design of BIMI", July 2020,
              <https://tools.ietf.org/html/draft-bkl-bimi-overview-00>.

Appendix A.  Example Selector Discovery (INFORMATIVE)

   This section shows several examples of how a receiving MTA should
   determine which Assertion Record to use depending on the BIMI-
   Selector header.

A.1.  No BIMI-Selector Header

   The domain example.com does not send with a BIMI-Selector header.

   From: sender@example.com

   The MTA would lookup default._bimi.example.com for the BIMI DNS
   record.

A.2.  With BIMI-Selector Header

   The domain example.com sends with a BIMI-Selector header:

   From: sender@example.com
   BIMI-Selector: v=BIMI1; s=selector;

   The MTA would lookup selector._bimi.example.com.

A.3.  Without BIMI-Selector Header on a subdomain

   The domain foo.example.com sends without a BIMI-Selector header:

   From: sender@foo.example.com

   The MTA would lookup default._bimi.foo.example.com for the BIMI DNS
   record.  If it did not exist, it would lookup
   default._bimi.example.com.

A.4.  With BIMI-Selector Header on a subdomain

   The domain foo.example.com sends without a BIMI-Selector header:

   From: sender@foo.example.com
   BIMI-Selector: v=BIMI1; s=selector;

   The MTA would lookup selector._bimi.foo.example.com for the BIMI DNS
   record.  If it did not exist, it would fall back to the lookup
   default._bimi.example.com.

A.5.  Invalid BIMI-Selector Header

   The domain example.com sends with a BIMI-Selector header, but does
   not include the required field 'v=':

   From: sender@example.com
   BIMI-Selector: s=selector;

   The MTA would ignore this header, and lookup
   default._bimi.example.com.

Appendix B.  Example Authentication-Results entry (INFORMATIONAL)

   This section shows example Authentication-Results stamps based on
   different BIMI lookup statuses.

B.1.  Successful BIMI lookup

   From: sender@example.com
   BIMI-Selector: v=BIMI1; s=selector;
   Authentication-Results: bimi=pass header.d=example.com header.selector=selector;

B.2.  No BIMI record

   From: sender@sub.example.com
   Authentication-Results: bimi=none;

   In this example, sub.example.com does not have a BIMI record at
   default._bimi.sub.example.com, nor does default._bimi.example.com

B.3.  Subdomain has no default record, but organizational domain does

   From: sender@sub.example.com
   Authentication-Results: bimi=pass header.d=example.com header.selector=default;

B.4.  Subdomain has no record for selector, but organization domain has a
default

   From: sender@sub.example.com
   BIMI-Selector: v=BIMI1; s=selector;
   Authentication-Results: bimi=pass header.d=example.com header.selector=default;

   In this example, the sender specified a DNS record at
   selector._bimi.sub.example.com but it did not exist.  The fallback is
   to use default._bimi.example.com, not selector._bimi.example.com even
   if that record exists.

Appendix C.  Example BIMI Headers Construction (INFORMATIONAL)

   This section shows how an example MTA might evaluate an incoming
   email for BIMI participation, and how it could share that
   determination with its MUA(s).

C.1.  MTA Receives an email

   The MTA receives the following DKIM signed message:

   DKIM-Signature: v=1; s=myExample; d=example.com; h=From;BIMI-Selector;Date;bh=...;b=...
   From: sender@example.com
   BIMI-Selector: v=BIMI1; s=brand;
   BIMI-Location: image.example.com/bimi/logo/example-bimi.svg
   Subject: Hi, this is a message from the good folks at Example Learning

C.2.  MTA does its authentication checks

   The receiving MTA receives the message and performs an SPF
   verification (which fails), a DKIM verification (which passes), and a
   DMARC verification (which passes).  The domain is verified and has
   good reputation.  The Receiver proceeds to perform a BIMI lookup.

C.3.  MTA performs BIMI Assertion

   The MTA sees that the message has a BIMI-Selector header, and it is
   covered by the DKIM-Signature, and the DKIM-Signature that passed
   DKIM is the one that covers the BIMI-Selector header.  The MTA sees
   the header validates and contains 'v=BIMI1', and 's=brand'.  It
   performs a DNS query for brand._bimi.example.com and retrieves:

   brand._bimi.example.com IN TXT "v=BIMI1; l=https://image.example.com/bimi/logo/"

   The MTA verifies the syntax of the BIMI DNS record, and it, too
   passes.

   The MTA knows it has previously retrieved the Indicator referenced by
   the BIMI DNS record, and had already successfully checked this
   Indicator against the published SVG profile.  The MTA retrieves the
   Indicator from the cache.

C.4.  MTA appends to Authentication-Results

   The MTA computes and affixes the results of the BIMI to the
   Authentication-Results header:

   Authentication-Results: spf=fail smtp.mailfrom=example.com;
     dkim=pass (signature was verified) header.d=example.com;
     dmarc=pass header.from=example.com;
     bimi=pass header.d=example.com header.selector=brand;

C.5.  MTA Constructs BIMI-Location and BIMI-Indicator headers

   The MTA base64 encodes the retrieved Indicator and constructs a new
   BIMI-Indicator header.

   The MTA constructs a BIMI-Location header with a version tag, and an
   l tag indicating the URL from which the Indicator was retrieved.

   Finally, the MTA removes any existing BIMI-Location and BIMI-
   Indicator headers, and stamps the new ones:

   BIMI-Location: v=BIMI1; l=https://image.example.com/bimi/logo/

   BIMI-Indicator: PD94bW...8L3N2Zz4K

   That the original sender signed a BIMI-Location header against this
   spec is irrelevant.  It was used for DKIM validation and then thrown
   out by the MTA.

C.6.  The MUA displays the Indicator

   The mail is opened from the mail store in an MUA.  The MUA performs
   locally defined checks to make sure that it can trust the BIMI-
   Indicator header.  Finally, the MUA extracts the Indicator from the
   BIMI-Indicator header and displays it to the user.

C.7.  Acknowledgements

   Many people have contributed to the development of BIMI.  Along with
   thanks to members of the current AuthIndicators Working Group, the
   editors wish to acknowledge the efforts of Sri Somanchi, Don
   Cardinal, Steve Jones, and John Levine.

Authors' Addresses

   Seth Blank
   Valimail

   Email: seth@valimail.com

   Peter Goldstein
   Valimail

   Email: peter@valimail.com


   Thede Loder
   Skyelogic Works LLC

   Email: thede@skyelogicworks.com


   Terry Zink
   Zink Magical Contraptions

   Email: tzink@terryzink.com


   Marc Bradshaw
   Fastmail

   Email: marc@fastmailteam.com


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