DMARC Working Group                                         M. Kucherawy
Internet-Draft                                           August 22,                                         November 14, 2018
Updates: 7601 (if approved)
Intended status: Standards Track
Expires: February 23, May 18, 2019

   Message Header Field for Indicating Message Authentication Status


   This document specifies a message header field called Authentication-
   Results for use with electronic mail messages to indicate the results
   of message authentication efforts.  Any receiver-side software, such
   as mail filters or Mail User Agents (MUAs), can use this header field
   to relay that information in a convenient and meaningful way to users
   or to make sorting and filtering decisions.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Trust Boundary . . . . . . . . . . . . . . . . . . . . . .  6
     1.3.  Processing Scope . . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
     1.5.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  7
       1.5.1.  Key Words  . . . . . . . . . . . . . . . . . . . . . .  7
       1.5.2.  Internationalized Email  . . . . . . . . . . . . . . .  7
       1.5.3.  Security . . . . . . . . . . . . . . . . . . . . . . .  7
       1.5.4.  Email Architecture . . . . . . . . . . . . . . . . . .  8
       1.5.5.  Other Terms  . . . . . . . . . . . . . . . . . . . . .  9
     1.6.  Trust Environment  . . . . . . . . . . . . . . . . . . . .  9
   2.  Definition and Format of the Header Field  . . . . . . . . . .  9
     2.1.  General Description  . . . . . . . . . . . . . . . . . . .  9
     2.2.  Formal Definition  . . . . . . . . . . . . . . . . . . . . 10
     2.3.  Property Types (ptypes) and Properties . . . . . . . . . . 13
     2.4.  The "policy" ptype . . . . . . . . . . . . . . . . . . . . 14
     2.5.  Authentication Identifier Field  . . . . . . . . . . . . . 14
     2.6.  Version Tokens . . . . . . . . . . . . . . . . . . . . . . 16
     2.7.  Defined Methods and Result Values  . . . . . . . . . . . . 16
       2.7.1.  DKIM and DomainKeys  . . . . . . . . . . . . . . . . . 16
       2.7.2.  SPF and Sender ID  . . . . . . . . . . . . . . . . . . 18
       2.7.3.  "iprev"  . . . . . . . . . . . . . . . . . . . . . . . 19
       2.7.4.  SMTP AUTH  . . . . . . . . . . . . . . . . . . . . . . 20
       2.7.5.  Other Registered Codes . . . . . . . . . . . . . . . . 21
       2.7.6.  Extension Methods  . . . . . . . . . . . . . . . . . . 21
       2.7.7.  Extension Result Codes . . . . . . . . . . . . . . . . 22
   3.  The "iprev" Authentication Method  . . . . . . . . . . . . . . 23
   4.  Adding the Header Field to a Message . . . . . . . . . . . . . 24
     4.1.  Header Field Position and Interpretation . . . . . . . . . 25
     4.2.  Local Policy Enforcement . . . . . . . . . . . . . . . . . 26
   5.  Removing Existing Header Fields  . . . . . . . . . . . . . . . 27
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
     6.1.  The Authentication-Results Header Field  . . . . . . . . . 28
     6.2.  "Email Authentication Methods" Registry Description  . . . 28
     6.3.  "Email Authentication Methods" Registry Update . . . . . . 28
       6.3.1.  'header.a' for DKIM  . . . . . . . . . . . . . . . . . 29
       6.3.2.  'header.s' for DKIM  . . . . . . . . . . . . . . . . . 29
     6.4.  "Email Authentication Property Types" Registry . . . . . . 29
     6.5.  "Email Authentication Result Names" Description  . . . . . 29
     6.6.  "Email Authentication Result Names" Update . . . . . . . . 30
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
     7.1.  Forged Header Fields . . . . . . . . . . . . . . . . . . . 30
     7.2.  Misleading Results . . . . . . . . . . . . . . . . . . . . 32
     7.3.  Header Field Position  . . . . . . . . . . . . . . . . . . 32
     7.4.  Reverse IP Query Denial-of-Service Attacks . . . . . . . . 32
     7.5.  Mitigation of Backscatter  . . . . . . . . . . . . . . . . 32
     7.6.  Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 32
     7.7.  Attacks against Authentication Methods . . . . . . . . . . 33
     7.8.  Intentionally Malformed Header Fields  . . . . . . . . . . 33
     7.9.  Compromised Internal Hosts . . . . . . . . . . . . . . . . 33
     7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 33
     7.11. Reverse Mapping  . . . . . . . . . . . . . . . . . . . . . 34
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 34
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 36
   Appendix A.  Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 38
   Appendix B.  Authentication-Results Examples . . . . . . . . . . . 38
     B.1.  Trivial Case; Header Field Not Present . . . . . . . . . . 39
     B.2.  Nearly Trivial Case; Service Provided, but No
           Authentication Done  . . . . . . . . . . . . . . . . . . . 39
     B.3.  Service Provided, Authentication Done  . . . . . . . . . . 40
     B.4.  Service Provided, Several Authentications Done, Single
           MTA  . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     B.5.  Service Provided, Several Authentications Done,
           Different MTAs . . . . . . . . . . . . . . . . . . . . . . 42
     B.6.  Service Provided, Multi-tiered Authentication Done . . . . 44
     B.7.  Comment-Heavy Example  . . . . . . . . . . . . . . . . . . 45
   Appendix C.  Operational Considerations about Message
                Authentication  . . . . . . . . . . . . . . . . . . . 46
   Appendix D.  Changes Since RFC7601 . . . . . . . . . . . . . . . . 47
   Appendix E.  Acknowledgments . . . . . . . . . . . . . . . . . . . 47
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 48

1.  Introduction

   This document describes a header field called Authentication-Results
   for electronic mail messages that presents the results of a message
   authentication effort in a machine-readable format.  The intent of
   the header field is to create a place to collect such data when
   message authentication mechanisms are in use so that a Mail User
   Agent (MUA) and downstream filters can make filtering decisions
   and/or provide a recommendation to the user as to the validity of the
   message's origin and possibly the safety and integrity of its

   End users are not expected to be direct consumers of this header
   field.  This header field is intended for consumption by programs
   that will then use such data or render it in a human-usable form.

   This document specifies the format of this header field and discusses
   the implications of its presence or absence.  However, it does not
   discuss how the data contained in the header field ought to be used,
   such as what filtering decisions are appropriate or how an MUA might
   render those results, as these are local policy and/or user interface
   design questions that are not appropriate for this document.

   At the time of publication of this document, the following are
   published email authentication methods:

   o  Author Domain Signing Practices ([ADSP]) (Historic)

   o  SMTP Service Extension for Authentication ([AUTH])

   o  DomainKeys Identified Mail Signatures ([DKIM])

   o  Domain-based Message Authentication, Reporting and Conformance

   o  Sender Policy Framework ([SPF])

   o  reverse IP address name validation ("iprev", defined in Section 3)

   o  Require-Recipient-Valid-Since Header Field and SMTP Service
      Extension ([RRVS])

   o  S/MIME Signature Verification ([SMIME-REG])

   o  Vouch By Reference ([VBR])

   o  DomainKeys ([DOMAINKEYS]) (Historic)
   o  Sender ID ([SENDERID]) (Experimental)

   There exist registries for tokens used within this header field that
   refer to the specifications listed above.  Section 6 describes the
   registries and their contents and specifies the process by which
   entries are added or updated.  It also updates the existing contents
   to match the current states of these specifications.

   The goal of this work is to give current and future authentication
   schemes a common framework within which to deliver their results to
   downstream agents and discourage the creation of unique header fields
   for each.

   Although SPF defined a header field called "Received-SPF" and the
   historic DomainKeys defined one called "DomainKey-Status" for this
   purpose, those header fields are specific to the conveyance of their
   respective results only and thus are insufficient to satisfy the
   requirements enumerated below.  In addition, many SPF implementations
   have adopted the header field specified here at least as an option,
   and DomainKeys has been obsoleted by DKIM.

1.1.  Purpose

   The header field defined in this document is expected to serve
   several purposes:

   1.  Convey the results of various message authentication checks,
       which are applied by upstream filters and Mail Transfer Agents
       (MTAs) and then passed to MUAs and downstream filters within the
       same "trust domain".  Such agents might wish to render those
       results to end users or to use those data to apply more or less
       stringent content checks based on authentication results;

   2.  Provide a common location within a message for this data;

   3.  Create an extensible framework for reporting new authentication
       methods as they emerge.

   In particular, the mere presence of this header field does not mean
   its contents are valid.  Rather, the header field is reporting
   assertions made by one or more authentication schemes applied
   somewhere upstream.  For an MUA or downstream filter to treat the
   assertions as actually valid, there must be an assessment of the
   trust relationship among such agents, the validating MTA, and the
   mechanism for conveying the information.

1.2.  Trust Boundary

   This document makes several references to the "trust boundary" of an
   administrative management domain (ADMD).  Given the diversity among
   existing mail environments, a precise definition of this term isn't

   Simply put, a transfer from the producer of the header field to the
   consumer must occur within a context that permits the consumer to
   treat assertions by the producer as being reliable and accurate
   (trustworthy).  How this trust is obtained is outside the scope of
   this document.  It is entirely a local matter.

   Thus, this document defines a "trust boundary" as the delineation
   between "external" and "internal" entities.  Services that are
   internal -- within the trust boundary -- are provided by the ADMD's
   infrastructure for its users.  Those that are external are outside of
   the authority of the ADMD.  By this definition, hosts that are within
   a trust boundary are subject to the ADMD's authority and policies,
   independent of their physical placement or their physical operation.
   For example, a host within a trust boundary might actually be
   operated by a remote service provider and reside physically within
   its data center.

   It is possible for a message to be evaluated inside a trust boundary
   but then depart and re-enter the trust boundary.  An example might be
   a forwarded message such as a message/rfc822 attachment (see
   Multipurpose Internet Mail Extensions [MIME]) or one that is part of
   a multipart/digest.  The details reported by this field cannot be
   trusted in that case.  Thus, this field found within one of those
   media types is typically ignored.

1.3.  Processing Scope

   The content of this header field is meant to convey to message
   consumers that authentication work on the message was already done
   within its trust boundary, and those results are being presented.  It
   is not intended to provide message parameters to consumers so that
   they can perform authentication protocols on their own.

1.4.  Requirements

   This document establishes no new requirements on existing protocols
   or servers.

   In particular, this document establishes no requirement on MTAs to
   reject or filter arriving messages that do not pass authentication
   checks.  The data conveyed by the specified header field's contents
   are for the information of MUAs and filters and are to be used at
   their discretion.

1.5.  Definitions

   This section defines various terms used throughout this document.

1.5.1.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [KEYWORDS].

1.5.2.  Internationalized Email

   In this document, there are references to messages formatted to
   support Email Address Internationalization (EAI).  Reference material
   for this can be found in [RFC6530], [RFC6531], and [RFC6532].
   Generally speaking, these documents allow UTF-8 in most places that
   free-form text can be found and U-labels where domain names can be
   used, and this document extends Authentication-Results accordingly.

1.5.3.  Security

   "Guidelines for Writing RFC Text on Security Considerations"
   ([SECURITY]) discusses authentication and authorization and the
   conflation of the two concepts.  The use of those terms within the
   context of recent message security work has given rise to slightly
   different definitions, and this document reflects those current
   usages, as follows:

   o  "Authorization" is the establishment of permission to use a
      resource or represent an identity.  In this context, authorization
      indicates that a message from a particular ADMD arrived via a
      route the ADMD has explicitly approved.

   o  "Authentication" is the assertion of validity of a piece of data
      about a message (such as the sender's identity) or the message in
      its entirety.

   As examples: SPF and Sender ID are authorization mechanisms in that
   they express a result that shows whether or not the ADMD that
   apparently sent the message has explicitly authorized the connecting
   Simple Mail Transfer Protocol ([SMTP]) client to relay messages on
   its behalf, but they do not actually validate any other property of
   the message itself.  By contrast, DKIM is agnostic as to the routing
   of a message but uses cryptographic signatures to authenticate
   agents, assign (some) responsibility for the message (which implies
   authorization), and ensure that the listed portions of the message
   were not modified in transit.  Since the signatures are not tied to
   SMTP connections, they can be added by either the ADMD of origin,
   intermediate ADMDs (such as a mailing list server), other handling
   agents, or any combination.

   Rather than create a separate header field for each class of
   solution, this specification groups them both into a single header

1.5.4.  Email Architecture

   o  A "border MTA" is an MTA that acts as a gateway between the
      general Internet and the users within an organizational boundary.
      (See also Section 1.2.)

   o  A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that
      actually enacts delivery of a message to a user's inbox or other
      final delivery.

   o  An "intermediate MTA" is any MTA that is not a delivery MTA and is
      also not the first MTA to handle the message.

   The following diagram illustrates the flow of mail among these
   defined components.  See Internet Mail Architecture [EMAIL-ARCH] for
   further discussion on general email system architecture, which
   includes detailed descriptions of these components, and Appendix C of
   this document for discussion about the common aspects of email
   authentication in current environments.

                          +-----+   +-----+   +------------+
                          | MUA |-->| MSA |-->| Border MTA |
                          +-----+   +-----+   +------------+
                                               | Internet |
   +-----+   +-----+   +------------------+   +------------+
   | MUA |<--| MDA |<--| Intermediate MTA |<--| Border MTA |
   +-----+   +-----+   +------------------+   +------------+

   Generally, it is assumed that the work of applying message
   authentication schemes takes place at a border MTA or a delivery MTA.

   This specification is written with that assumption in mind.  However,
   there are some sites at which the entire mail infrastructure consists
   of a single host.  In such cases, such terms as "border MTA" and
   "delivery MTA" might well apply to the same machine or even the very
   same agent.  It is also possible that some message authentication
   tests could take place on an intermediate MTA.  Although this
   document doesn't specifically describe such cases, they are not meant
   to be excluded.

1.5.5.  Other Terms

   In this document, the term "producer" refers to any component that
   adds this header field to messages it is handling, and "consumer"
   refers to any component that identifies, extracts, and parses the
   header field to use as part of a handling decision.

1.6.  Trust Environment

   This header field permits one or more message validation mechanisms
   to communicate output to one or more separate assessment mechanisms.
   These mechanisms operate within a unified trust boundary that defines
   an Administrative Management Domain (ADMD).  An ADMD contains one or
   more entities that perform validation and generate the header field
   and one or more that consume it for some type of assessment.  The
   field often contains no integrity or validation mechanism of its own,
   so its presence must be trusted implicitly.  Hence, valid use of the
   header field requires removing any occurrences of it that claim to be
   associated with the ADMD when the message enters the ADMD.  This
   ensures that later occurrences have been added within the trust
   boundary of the ADMD.

   The authserv-id token defined in Section 2.2 can be used to reference
   an entire ADMD or a specific validation engine within an ADMD.
   Although the labeling scheme is left as an operational choice, some
   guidance for selecting a token is provided in later sections of this

2.  Definition and Format of the Header Field

   This section gives a general overview of the format of the header
   field being defined and then provides a formal specification.

2.1.  General Description

   The header field specified here is called Authentication-Results.  It
   is a Structured Header Field as defined in Internet Message Format
   ([MAIL]), and thus all of the related definitions in that document

   This header field is added at the top of the message as it transits
   MTAs that do authentication checks, so some idea of how far away the
   checks were done can be inferred.  It is therefore considered to be a
   trace field as defined in [MAIL], and thus all of the related
   definitions in that document apply.

   The value of the header field (after removing comments) consists of
   an authentication identifier, an optional version, and then a series
   of statements and supporting data.  The statements are of the form
   "method=result" and indicate which authentication method(s) were
   applied and their respective results.  For each such statement, the
   supporting data can include a "reason" string and one or more
   "property=value" statements indicating which message properties were
   evaluated to reach that conclusion.

   The header field can appear more than once in a single message, more
   than one result can be represented in a single header field, or a
   combination of these can be applied.

2.2.  Formal Definition

   Formally, the header field is specified as follows using Augmented
   Backus-Naur Form ([ABNF]):

     authres-header-field = "Authentication-Results:" authres-payload

     authres-payload = [CFWS] authserv-id
                       [ CFWS authres-version ]
                       ( no-result / 1*resinfo ) [CFWS] CRLF

     authserv-id = value
                 ; see below for a description of this element

     authres-version = 1*DIGIT [CFWS]
             ; indicates which version of this specification is in use;
             ; this specification is version "1", and the absence of a
             ; version implies this version of the specification

     no-result = [CFWS] ";" [CFWS] "none"
               ; the special case of "none" is used to indicate that no
               ; message authentication was performed

     resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
               *( CFWS propspec )

     methodspec = [CFWS] method [CFWS] "=" [CFWS] result
                ; indicates which authentication method was evaluated
                ; and what its output was

     reasonspec = "reason" [CFWS] "=" [CFWS] value
                ; a free-form comment on the reason the given result
                ; was returned

     propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue
              ; an indication of which properties of the message
              ; were evaluated by the authentication scheme being
              ; applied to yield the reported result

     method = Keyword [ [CFWS] "/" [CFWS] method-version ]
            ; a method indicates which method's result is
            ; represented by "result", and is one of the methods
            ; explicitly defined as valid in this document
            ; or is an extension method as defined below

     method-version = 1*DIGIT [CFWS]
            ; indicates which version of the method specification is
            ; in use, corresponding to the matching entry in the IANA
            ; "Email Authentication Methods" registry; a value of "1"
            ; is assumed if this version string is absent

     result = Keyword
            ; indicates the results of the attempt to authenticate
            ; the message; see below for details

     ptype = Keyword
           ; indicates whether the property being evaluated was
           ; a parameter to an [SMTP] command, was a value taken
           ; from a message header field, was some property of
           ; the message body, or was some other property evaluated by
           ; the receiving MTA; expected to be one of the "property
           ; types" explicitly defined as valid, or an extension
           ; ptype, as defined below

     property = special-smtp-verb / Keyword
             ; indicates more specifically than "ptype" what the
             ; source of the evaluated property is; the exact meaning
             ; is specific to the method whose result is being reported
             ; and is defined more clearly below

     special-smtp-verb = "mailfrom" / "rcptto"
             ; special cases of [SMTP] commands that are made up
             ; of multiple words

     pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )

            ; the value extracted from the message property defined
            ; by the "" construction

   "local-part" is defined in Section 3.4.1 of [MAIL], as modified by

   "CFWS" is defined in Section 3.2.2 of [MAIL].

   "domain-name" is as defined in Section 3.5 of [DKIM] where the "d="
   tag is defined, with "sub-domain" as modified by [RFC6531].

   "Keyword" is defined in Section 4.1.2 of [SMTP].  It is further
   constrained by the necessity of being registered in the IANA registry
   relevant to the context in which it is used.  See Section 2.7,
   Section 2.3, and Section 6.

   "value" is as defined in Section 5.1 of [MIME]. [MIME], with "quoted-string"
   updated as specified in RFC 6532.

   See Section 2.5 for a description of the authserv-id element.

   If the value portion of a "pvalue" construction identifies something
   intended to be an email identity, then it MUST use the right hand
   portion of that ABNF definition.

   The list of commands eligible for use with the "smtp" ptype can be
   found in Section 4.1 of [SMTP].

   The "propspec" may be omitted if, for example, the method was unable
   to extract any properties to do its evaluation yet still has a result
   to report.  It may also be omitted if the agent generating this
   result wishes not to reveal such properties to downstream agents.

   Where an SMTP command name is being reported as a "property", the
   agent generating the header field represents that command by
   converting it to lowercase and dropping any spaces (e.g., "MAIL FROM"
   becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.).

   A "ptype" value of "policy" indicates a policy decision about the
   message not specific to a property of the message that could be
   extracted.  See Section 2.4 for details.

   Examples of complete messages using this header field can be found in
   Appendix B.

2.3.  Property Types (ptypes) and Properties

   The "ptype" in the ABNF above indicates the general type of property
   being described by the result being reported, upon which the reported
   result was based.  Coupled with the "property", which is more
   specific, they indicate from where the reported data were extracted.
   This can include part of the message header or body, some part of the
   SMTP session, a secondary output of an authentication method (apart
   from its pure result), or some other aspect of the message's

   Combinations of ptypes and properties are registered and described in
   the "Email Authentication Methods" registry, coupled with the
   authentication methods with which they are used.  This is further
   described in Section 6.

   Legal values of "ptype" are as defined in the IANA "Email
   Authentication Property Types" registry, created by [RFC7410].  The
   initial values and what they typically indicate are as follows, based
   on [RFC7001]:

   body:  Information that was extracted from the body of the message.
      This might be an arbitrary string of bytes, a hash of a string of
      bytes, a Uniform Resource Identifier, or some other content of
      interest.  The "property" is an indication of where within the
      message body the extracted content was found, and can indicate an
      offset, identify a MIME part, etc.

   header:  Indicates information that was extracted from the header of
      the message.  This might be the value of a header field or some
      portion of a header field.  The "property" gives a more precise
      indication of the place in the header from which the extraction
      took place.

   policy:  A local policy mechanism was applied that augments or
      overrides the result returned by the authentication mechanism.
      (See Section 2.4.)

   smtp:  Indicates information that was extracted from an SMTP command
      that was used to relay the message.  The "property" indicates
      which SMTP command included the extracted content as a parameter.

   Results reported using unknown ptypes MUST NOT be used in making
   handling decisions.  They can be safely ignored by consumers.

   Entries in the "Email Authentication Methods" registry can define
   properties that deviate from these definitions when appropriate.
   Such deviations need to be clear in the registry and/or in the
   defining document.  See Section 2.7.1 for an example.

2.4.  The "policy" ptype

   A special ptype value of "policy" is also defined.  This ptype is
   provided to indicate that some local policy mechanism was applied
   that augments or even replaces (i.e., overrides) the result returned
   by the authentication mechanism.  The property and value in this case
   identify the local policy that was applied and the result it

   For example, a DKIM signature is not required to include the Subject
   header field in the set of fields that are signed.  An ADMD receiving
   such a message might decide that such a signature is unacceptable,
   even if it passes, because the content of the Subject header field
   could be altered post-signing without invalidating the signature.
   Such an ADMD could replace the DKIM "pass" result with a "policy"
   result and then also include the following in the corresponding
   Authentication-Result field:

      ... dkim=policy policy.dkim-rules=unsigned-subject ...

   In this case, the property is "dkim-rules", indicating some local
   check by that name took place and that check returned a result of
   "unsigned-subject".  These are arbitrary names selected by (and
   presumably used within) the ADMD making use of them, so they are not
   normally registered with IANA or otherwise specified apart from
   setting syntax restrictions that allow for easy parsing within the
   rest of the header field.

   This ptype existed in the original specification for this header
   field, but without a complete description or example of intended use.
   As a result, it has not seen any practical use to date that matches
   its intended purpose.  These added details are provided to guide
   implementers toward proper use.

2.5.  Authentication Identifier Field

   Every Authentication-Results header field has an authentication
   service identifier field (authserv-id above).  Specifically, this is
   any string intended to identify the authentication service within the
   ADMD that conducted authentication checks on the message.  This
   identifier is intended to be machine-readable and not necessarily
   meaningful to users.

   Note that in an EAI-formatted message, this identifier may be
   expressed in UTF-8.

   Since agents consuming this field will use this identifier to
   determine whether its contents are of interest (and are safe to use),
   the uniqueness of the identifier MUST be guaranteed by the ADMD that
   generates it and MUST pertain to that ADMD.  MUAs or downstream
   filters SHOULD use this identifier to determine whether or not the
   data contained in an Authentication-Results header field ought to be
   used or ignored.

   For simplicity and scalability, the authentication service identifier
   SHOULD be a common token used throughout the ADMD.  Common practice
   is to use the DNS domain name used by or within that ADMD, sometimes
   called the "organizational domain", but this is not strictly

   For tracing and debugging purposes, the authentication identifier can
   instead be the specific hostname of the MTA performing the
   authentication check whose result is being reported.  Moreover, some
   implementations define a substructure to the identifier; such
   structures are outside of the scope of this specification.

   Note, however, that using a local, relative identifier like a flat
   hostname, rather than a hierarchical and globally unique ADMD
   identifier like a DNS domain name, makes configuration more difficult
   for large sites.  The hierarchical identifier permits aggregating
   related, trusted systems together under a single, parent identifier,
   which in turn permits assessing the trust relationship with a single
   reference.  The alternative is a flat namespace requiring
   individually listing each trusted system.  Since consumers will use
   the identifier to determine whether to use the contents of the header

   o  Changes to the identifier impose a large, centralized
      administrative burden.

   o  Ongoing administrative changes require constantly updating this
      centralized table, making it difficult to ensure that an MUA or
      downstream filter will have access to accurate information for
      assessing the usability of the header field's content.  In
      particular, consumers of the header field will need to know not
      only the current identifier(s) in use but previous ones as well to
      account for delivery latency or later re-assessment of the header
      field's contents.

   Examples of valid authentication identifiers are "",
   "", "", and "example-auth".

2.6.  Version Tokens

   The grammar above provides for the optional inclusion of versions on
   both the header field itself (attached to the authserv-id token) and
   on each of the methods being reported.  The method version refers to
   the method itself, which is specified in the documents describing
   those methods, while the authserv-id version refers to this document
   and thus the syntax of this header field.

   The purpose of including these is to avoid misinterpretation of the
   results.  That is, if a parser finds a version after an authserv-id
   that it does not explicitly know, it can immediately discontinue
   trying to parse since what follows might not be in an expected
   format.  For a method version, the parser SHOULD ignore a method
   result if the version is not supported in case the semantics of the
   result have a different meaning than what is expected.  For example,
   if a hypothetical DKIM version 2 yielded a "pass" result for
   different reasons than version 1 does, a consumer of this field might
   not want to use the altered semantics.  Allowing versions in the
   syntax is a way to indicate this and let the consumer of the header
   field decide.

2.7.  Defined Methods and Result Values

   Each individual authentication method returns one of a set of
   specific result values.  The subsections below provide references to
   the documents defining the authentication methods specifically
   supported by this document, and their corresponding result values.
   Verifiers SHOULD use these values as described below.  New methods
   not specified in this document, but intended to be supported by the
   header field defined here, MUST include a similar result table either
   in their defining documents or in supplementary ones.

2.7.1.  DKIM and DomainKeys

   DKIM is represented by the "dkim" method and is defined in [DKIM].
   DomainKeys is defined in [DOMAINKEYS] and is represented by the
   "domainkeys" method.

   Section 3.8 of [DOMAINKEYS] enumerates some possible results of a
   DomainKeys evaluation.  Those results are not used when generating
   this header field; rather, the results returned are listed below.

   A signature is "acceptable to the ADMD" if it passes local policy
   checks (or there are no specific local policy checks).  For example,
   an ADMD policy might require that the signature(s) on the message be
   added using the DNS domain present in the From header field of the
   message, thus making third-party signatures unacceptable even if they

   Both DKIM and DomainKeys use the same result set, as follows:

   none:  The message was not signed.

   pass:  The message was signed, the signature or signatures were
      acceptable to the ADMD, and the signature(s) passed verification

   fail:  The message was signed and the signature or signatures were
      acceptable to the ADMD, but they failed the verification test(s).

   policy:  The message was signed, but some aspect of the signature or
      signatures was not acceptable to the ADMD.

   neutral:  The message was signed, but the signature or signatures
      contained syntax errors or were not otherwise able to be
      processed.  This result is also used for other failures not
      covered elsewhere in this list.

   temperror:  The message could not be verified due to some error that
      is likely transient in nature, such as a temporary inability to
      retrieve a public key.  A later attempt may produce a final

   permerror:  The message could not be verified due to some error that
      is unrecoverable, such as a required header field being absent.  A
      later attempt is unlikely to produce a final result.

   DKIM results are reported using a ptype of "header".  The property,
   however, represents one of the tags found in the DKIM-Signature
   header field rather than a distinct header field.  For example, the
   ptype-property combination "header.d" refers to the content of the
   "d" (signing domain) tag from within the signature header field, and
   not a distinct header field called "d".

   Note that in an EAI-formatted message, the values of the "d" and "i"
   properties can be expressed in UTF-8.

   In addition to previous registrations, this document registers the
   DKIM tag "a" (cryptographic algorithm used to sign the message) as a
   reportable property.  This can be used to aid receivers during post-
   verification processing.  In particular, [RFC8301] obsoleted use of
   the "rsa-sha1" algorithm in DKIM, so it is important to be able to
   distinguish such signatures from those using preferred algorithms.

   The ability to report different DKIM results for a message with
   multiple signatures is described in [RFC6008].

   [DKIM] advises that if a message fails verification, it is to be
   treated as an unsigned message.  A report of "fail" here permits the
   receiver of the report to decide how to handle the failure.  A report
   of "neutral" or "none" preempts that choice, ensuring the message
   will be treated as if it had not been signed.

   Section 3.1 of [DOMAINKEYS] describes a process by which the sending
   address of the message is determined.  DomainKeys results are thus
   reported along with the signing domain name, the sending address of
   the message, and the name of the header field from which the latter
   was extracted.  This means that a DomainKeys result includes a ptype-
   property combination of "header.d", plus one of "header.from" and
   "header.sender".  The sending address extracted from the header is
   included with any [MAIL]-style comments removed; moreover, the local-
   part of the address and the "@" character are removed if it has not
   been authenticated in some way.

2.7.2.  SPF and Sender ID

   SPF and Sender ID use the "spf" and "sender-id" method names,
   respectively.  The result values for SPF are defined in Section 2.6
   of [SPF], and those definitions are included here by reference:

     |    Code   | Meaning                        |
     | none      | [RFC7208], Section 2.6.1       |
     | pass      | [RFC7208], Section 2.6.3       |
     | fail      | [RFC7208], Section 2.6.4       |
     | softfail  | [RFC7208], Section 2.6.5       |
     | policy    | RFC 7601, Section 2.4          |
     | neutral   | [RFC7208], Section 2.6.2       |
     | temperror | [RFC7208], Section 2.6.6       |
     | permerror | [RFC7208], Section 2.6.7       |

   These result codes are used in the context of this specification to
   reflect the result returned by the component conducting SPF

   For SPF, the ptype used is "smtp", and the property is either
   "mailfrom" or "helo", since those values are the ones SPF can
   evaluate.  (If the SMTP client issued the EHLO command instead of
   HELO, the property used is "helo".)

   Note that in an EAI-formatted message, the "mailfrom" value can be
   expressed in UTF-8.

   The "sender-id" method is described in [SENDERID].  For this method,
   the ptype used is "header" and the property will be the name of the
   header field from which the Purported Responsible Address (see [PRA])
   was extracted -- namely, one of "Resent-Sender", "Resent-From",
   "Sender", or "From".

   The results for Sender ID are listed and described in Section 4.2 of
   [SENDERID], but for the purposes of this specification, the SPF
   definitions enumerated above are used instead.  Also, [SENDERID]
   specifies result codes that use mixed case, but they are typically
   used all lowercase in this context.

   For both methods, an additional result of "policy" is defined, which
   means the client was authorized to inject or relay mail on behalf of
   the sender's DNS domain according to the authentication method's
   algorithm, but local policy dictates that the result is unacceptable.
   For example, "policy" might be used if SPF returns a "pass" result,
   but a local policy check matches the sending DNS domain to one found
   in an explicit list of unacceptable DNS domains (e.g., spammers).

   If the retrieved sender policies used to evaluate SPF and Sender ID
   do not contain explicit provisions for authenticating the local-part
   (see Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported
   along with results for these mechanisms SHOULD NOT include the local-
   part or the following "@" character.

2.7.3.  "iprev"

   The result values used by the "iprev" method, defined in Section 3,
   are as follows:

   pass:  The DNS evaluation succeeded, i.e., the "reverse" and
      "forward" lookup results were returned and were in agreement.

   fail:  The DNS evaluation failed.  In particular, the "reverse" and
      "forward" lookups each produced results, but they were not in
      agreement, or the "forward" query completed but produced no
      result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an
      RCODE of 0 (NOERROR) in a reply containing no answers, was

   temperror:  The DNS evaluation could not be completed due to some
      error that is likely transient in nature, such as a temporary DNS
      error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or
      other error condition resulted.  A later attempt may produce a
      final result.

   permerror:  The DNS evaluation could not be completed because no PTR
      data are published for the connecting IP address, e.g., a DNS
      RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR)
      in a reply containing no answers, was returned.  This prevented
      completion of the evaluation.  A later attempt is unlikely to
      produce a final result.

   There is no "none" for this method since any TCP connection
   delivering email has an IP address associated with it, so some kind
   of evaluation will always be possible.

   The result is reported using a ptype of "policy" (as this is not part
   of any established protocol) and a property of "iprev".

   For discussion of the format of DNS replies, see "Domain Names -
   Implementation and Specification" ([DNS]).

2.7.4.  SMTP AUTH

   SMTP AUTH (defined in [AUTH]) is represented by the "auth" method.
   Its result values are as follows:

   none:  SMTP authentication was not attempted.

   pass:  The SMTP client authenticated to the server reporting the
      result using the protocol described in [AUTH].

   fail:  The SMTP client attempted to authenticate to the server using
      the protocol described in [AUTH] but was not successful (such as
      providing a valid identity but an incorrect password).

   temperror:  The SMTP client attempted to authenticate using the
      protocol described in [AUTH] but was not able to complete the
      attempt due to some error that is likely transient in nature, such
      as a temporary directory service lookup error.  A later attempt
      may produce a final result.

   permerror:  The SMTP client attempted to authenticate using the
      protocol described in [AUTH] but was not able to complete the
      attempt due to some error that is likely not transient in nature,
      such as a permanent directory service lookup error.  A later
      attempt is not likely to produce a final result.

   The result of AUTH is reported using a ptype of "smtp" and a property
   of either:

   o  "auth", in which case the value is the authorization identity
      generated by the exchange initiated by the AUTH command; or

   o  "mailfrom", in which case the value is the mailbox identified by
      the AUTH parameter used with the MAIL FROM command.  Note that in
      an EAI-formatted message, these values can be expressed in UTF-8.

   If both identities are available, both can be reported.  For example,
   consider this command issued by a client that has completed session
   authentication with the AUTH command resulting in an authorized
   identity of "client@c.example":

     MAIL FROM:<alice@a.example> AUTH=<bob@b.example>

   This could result in a "resinfo" construction like so:

     ; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example

   Note that in all cases other than "pass", the message was sent by an
   unauthenticated client.  All non-"pass" cases SHOULD thus be treated
   as equivalent with respect to this method.

2.7.5.  Other Registered Codes

   Result codes were also registered in other RFCs as follows:

   o  Vouch By Reference (in [AR-VBR], represented by "vbr");

   o  Authorized Third-Party Signatures (in [ATPS], represented by

   o  Author Domain Signing Practices (in [ADSP], represented by "dkim-

   o  Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs");

   o  S/MIME (in [SMIME-REG], represented by "smime").

   Note that "" and "", which are already registered, are
   permitted to be UTF-8 in an EAI-formatted message.

2.7.6.  Extension Methods

   Additional authentication method identifiers (extension methods) may
   be defined in the future by later revisions or extensions to this
   specification.  These method identifiers are registered with the
   Internet Assigned Numbers Authority (IANA) and, preferably, published
   in an RFC.  See Section 6 for further details.

   Extension methods can be defined for the following reasons:

   1.  To allow additional information from new authentication systems
       to be communicated to MUAs or downstream filters.  The names of
       such identifiers ought to reflect the name of the method being
       defined but ought not be needlessly long.

   2.  To allow the creation of "sub-identifiers" that indicate
       different levels of authentication and differentiate between
       their relative strengths, e.g., "auth1-weak" and "auth1-strong".

   Authentication method implementers are encouraged to provide adequate
   information, via message header field comments if necessary, to allow
   an MUA developer to understand or relay ancillary details of
   authentication results.  For example, if it might be of interest to
   relay what data was used to perform an evaluation, such information
   could be relayed as a comment in the header field, such as:

                  foo=pass bar.baz=blob (2 of 3 tests OK)

   Experimental method identifiers MUST only be used within ADMDs that
   have explicitly consented to use them.  These method identifiers and
   the parameters associated with them are not documented in RFCs.
   Therefore, they are subject to change at any time and not suitable
   for production use.  Any MTA, MUA, or downstream filter intended for
   production use SHOULD ignore or delete any Authentication-Results
   header field that includes an experimental (unknown) method

2.7.7.  Extension Result Codes

   Additional result codes (extension results) might be defined in the
   future by later revisions or extensions to this specification.  Non-
   experimental result codes MUST be registered with the Internet
   Assigned Numbers Authority (IANA) and preferably published in an RFC.
   See Section 6 for further details.

   Experimental results MUST only be used within ADMDs that have
   explicitly consented to use them.  These results and the parameters
   associated with them are not formally documented.  Therefore, they
   are subject to change at any time and not suitable for production
   use.  Any MTA, MUA, or downstream filter intended for production use
   SHOULD ignore or delete any Authentication-Results header field that
   includes an extension result.

3.  The "iprev" Authentication Method

   This section defines an additional authentication method called

   "iprev" is an attempt to verify that a client appears to be valid
   based on some DNS queries, which is to say that the IP address is
   explicitly associated with a domain name.  Upon receiving a session
   initiation of some kind from a client, the IP address of the client
   peer is queried for matching names (i.e., a number-to-name
   translation, also known as a "reverse lookup" or a "PTR" record
   query).  Once that result is acquired, a lookup of each of the names
   (i.e., a name-to-number translation, or an "A" or "AAAA" record
   query) thus retrieved is done.  The response to this second check
   will typically result in at least one mapping back to the client's IP

   Expressed as an algorithm: If the client peer's IP address is I, the
   list of names to which I maps (after a "PTR" query) is the set N, and
   the union of IP addresses to which each member of N maps (after
   corresponding "A" and "AAAA" queries) is L, then this test is
   successful if I is an element of L.

   Often an MTA receiving a connection that fails this test will simply
   reject the connection using the enhanced status code defined in
   [AUTH-ESC].  If an operator instead wishes to make this information
   available to downstream agents as a factor in handling decisions, it
   records a result in accordance with Section 2.7.3.

   The response to a PTR query could contain multiple names.  To prevent
   heavy DNS loads, agents performing these queries MUST be implemented
   such that the number of names evaluated by generation of
   corresponding A or AAAA queries is limited so as not to be unduly
   taxing to the DNS infrastructure, though it MAY be configurable by an
   administrator.  As an example, Section 4.6.4 of [SPF] chose a limit
   of 10 for its implementation of this algorithm.

   "DNS Extensions to Support IP Version 6" ([DNS-IP6]) discusses the
   query formats for the IPv6 case.

   There is some contention regarding the wisdom and reliability of this
   test.  For example, in some regions, it can be difficult for this
   test ever to pass because the practice of arranging to match the
   forward and reverse DNS is infrequently observed.  Therefore, the
   precise implementation details of how a verifier performs an "iprev"
   test are not specified here.  The verifier MAY report a successful or
   failed "iprev" test at its discretion having done some kind of check
   of the validity of the connection's identity using DNS.  It is
   incumbent upon an agent making use of the reported "iprev" result to
   understand what exactly that particular verifier is attempting to

   Extensive discussion of reverse DNS mapping and its implications can
   be found in "Considerations for the use of DNS Reverse Mapping"
   ([DNSOP-REVERSE]).  In particular, it recommends that applications
   avoid using this test as a means of authentication or security.  Its
   presence in this document is not an endorsement but is merely
   acknowledgment that the method remains common and provides the means
   to relay the results of that test.

4.  Adding the Header Field to a Message

   This specification makes no attempt to evaluate the relative
   strengths of various message authentication methods that may become
   available.  The methods listed are an order-independent set; their
   sequence does not indicate relative strength or importance of one
   method over another.  Instead, the MUA or downstream filter consuming
   this header field is to interpret the result of each method based on
   its own knowledge of what that method evaluates.

   Each "method" MUST refer to an authentication method declared in the
   IANA registry or an extension method as described in Section 2.7.6,
   and each "result" MUST refer to a result code declared in the IANA
   registry or an extension result code as defined in Section 2.7.7.
   See Section 6 for further information about the registered methods
   and result codes.

   An MTA compliant with this specification adds this header field
   (after performing one or more message authentication tests) to
   indicate which MTA or ADMD performed the test, which test was
   applied, and what the result was.  If an MTA applies more than one
   such test, it adds this header field either once per test or once
   indicating all of the results.  An MTA MUST NOT add a result to an
   existing header field.

   An MTA MAY add this header field containing only the authentication
   identifier portion and the "none" token (see Section 2.2) to indicate
   explicitly that no message authentication schemes were applied prior
   to delivery of this message.

   An MTA adding this header field has to take steps to identify it as
   legitimate to the MUAs or downstream filters that will ultimately
   consume its content.  One process to do so is described in Section 5.
   Further measures may be necessary in some environments.  Some
   possible solutions are enumerated in Section 7.1.  This document does
   not mandate any specific solution to this issue as each environment
   has its own facilities and limitations.

   Most known message authentication methods focus on a particular
   identifier to evaluate.  SPF and Sender ID differ in that they can
   yield a result based on more than one identifier; specifically, SPF
   can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom
   parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter
   or the Purported Responsible Address (PRA) identity.  When generating
   this field to report those results, only the parameter that yielded
   the result is included.

   For MTAs that add this header field, adding header fields in order
   (at the top), per Section 3.6 of [MAIL], is particularly important.
   Moreover, this header field SHOULD be inserted above any other trace
   header fields such MTAs might prepend.  This placement allows easy
   detection of header fields that can be trusted.

   End users making direct use of this header field might inadvertently
   trust information that has not been properly vetted.  If, for
   example, a basic SPF result were to be relayed that claims an
   authenticated addr-spec, the local-part of that addr-spec has
   actually not been authenticated.  Thus, an MTA adding this header
   field SHOULD NOT include any data that has not been authenticated by
   the method(s) being applied.  Moreover, MUAs SHOULD NOT render to
   users such information if it is presented by a method known not to
   authenticate it.

4.1.  Header Field Position and Interpretation

   In order to ensure non-ambiguous results and avoid the impact of
   false header fields, MUAs and downstream filters SHOULD NOT interpret
   this header field unless specifically configured to do so by the user
   or administrator.  That is, this interpretation should not be "on by
   default".  Naturally then, users or administrators ought not activate
   such a feature unless (1) they are certain the header field will be
   validly added by an agent within the ADMD that accepts the mail that
   is ultimately read by the MUA, and (2) instances of the header field
   that appear to originate within the ADMD but are actually added by
   foreign MTAs will be removed before delivery.

   Furthermore, MUAs and downstream filters SHOULD NOT interpret this
   header field unless the authentication service identifier of the
   header field is used within the ADMD as configured by the user or

   MUAs and downstream filters MUST ignore any result reported using a
   "result" not specified in the IANA "Result Code" registry or a
   "ptype" not listed in the "Email Authentication Property Types"
   registry for such values as defined in Section 6.  Moreover, such
   agents MUST ignore a result indicated for any "method" they do not
   specifically support.

   An MUA SHOULD NOT reveal these results to end users, absent careful
   human factors design considerations and testing, for the presentation
   of trust-related materials.  For example, an attacker could register (note the digit "1" (one)) and send signed mail to
   intended victims; a verifier would detect that the signature was
   valid and report a "pass" even though it's clear the DNS domain name
   was intended to mislead.  See Section 7.2 for further discussion.

   As stated in Section 2.1, this header field MUST be treated as though
   it were a trace header field as defined in Section 3.6.7 of [MAIL]
   and hence MUST NOT be reordered and MUST be prepended to the message,
   so that there is generally some indication upon delivery of where in
   the chain of handling MTAs the message authentication was done.

   Note that there are a few message handlers that are only capable of
   appending new header fields to a message.  Strictly speaking, these
   handlers are not compliant with this specification.  They can still
   add the header field to carry authentication details, but any signal
   about where in the handling chain the work was done may be lost.
   Consumers SHOULD be designed such that this can be tolerated,
   especially from a producer known to have this limitation.

   MUAs SHOULD ignore instances of this header field discovered within
   message/rfc822 MIME attachments.

   Further discussion of these topics can be found in Section 7 below.

4.2.  Local Policy Enforcement

   Some sites have a local policy that considers any particular
   authentication policy's non-recoverable failure results (typically
   "fail" or similar) as justification for rejecting the message.  In
   such cases, the border MTA SHOULD issue an SMTP rejection response to
   the message, rather than adding this header field and allowing the
   message to proceed toward delivery.  This is more desirable than
   allowing the message to reach an internal host's MTA or spam filter,
   thus possibly generating a local rejection such as a Delivery Status
   Notification (DSN) [DSN] to a forged originator.  Such generated
   rejections are colloquially known as "backscatter".

   The same MAY also be done for local policy decisions overriding the
   results of the authentication methods (e.g., the "policy" result
   codes described in Section 2.7).

   Such rejections at the SMTP protocol level are not possible if local
   policy is enforced at the MUA and not the MTA.

5.  Removing Existing Header Fields

   To mitigate the impact of forged header fields, any MTA conforming to
   this specification MUST delete any discovered instance of this header
   field that claims, by virtue of its authentication service
   identifier, to have been added within its trust boundary but that did
   not come directly from another trusted MTA.  For example, an MTA for receiving a message MUST delete or otherwise obscure any
   instance of this header field bearing an authentication service
   identifier indicating that the header field was added within prior to adding its own header fields.  This could mean
   each MTA will have to be equipped with a list of internal MTAs known
   to be compliant (and hence trustworthy).

   For messages that are EAI-formatted messages, this test is done after
   converting A-labels into U-labels.

   For simplicity and maximum security, a border MTA could remove all
   instances of this header field on mail crossing into its trust
   boundary.  However, this may conflict with the desire to access
   authentication results performed by trusted external service
   providers.  It may also invalidate signed messages whose signatures
   cover external instances of this header field.  A more robust border
   MTA could allow a specific list of authenticating MTAs whose
   information is to be admitted, removing the header field originating
   from all others.

   As stated in Section 1.2, a formal definition of "trust boundary" is
   deliberately not made here.  It is entirely possible that a border
   MTA for will explicitly trust authentication results
   asserted by upstream host even though they exist in
   completely disjoint administrative boundaries.  In that case, the
   border MTA MAY elect not to delete those results; moreover, the
   upstream host doing some authentication work could apply a signing
   technology such as [DKIM] on its own results to assure downstream
   hosts of their authenticity.  An example of this is provided in
   Appendix B.

   Similarly, in the case of messages signed using [DKIM] or other
   message-signing methods that sign header fields, this removal action
   could invalidate one or more signatures on the message if they
   covered the header field to be removed.  This behavior can be
   desirable since there's little value in validating the signature on a
   message with forged header fields.  However, signing agents MAY
   therefore elect to omit these header fields from signing to avoid
   this situation.

   An MTA SHOULD remove any instance of this header field bearing a
   version (express or implied) that it does not support.  However, an
   MTA MUST remove such a header field if the [SMTP] connection relaying
   the message is not from a trusted internal MTA.  This means the MTA
   needs to be able to understand versions of this header field at least
   as late as the ones understood by the MUAs or other consumers within
   its ADMD.

6.  IANA Considerations

   IANA has registered the defined header field and created registries
   as described below.  These registry actions were originally defined
   by [RFC5451] and updated by [RFC6577] and [RFC7001].  The created
   registries were further updated in [RFC7601] to make them more

   Each is listed below, though generally they are not changed by this

6.1.  The Authentication-Results Header Field

   The Authentication-Results header field was added to the IANA
   "Permanent Message Header Field Names" registry, per the procedure
   found in [IANA-HEADERS].  That entry will be updated to reference
   this document.  The following is the registration template:

     Header field name: Authentication-Results
     Applicable protocol: mail ([MAIL])
     Status: Standard
     Author/Change controller: IETF
     Specification document(s): [this document]
     Related information: none

6.2.  "Email Authentication Methods" Registry Description

   No changes are made to the description of this registry.

6.3.  "Email Authentication Methods" Registry Update

   The following two entries are added.

6.3.1.  'header.a' for DKIM

   Method:  dkim

   Definition:  [this document]

   ptype:  header

   property:  a

   Description:  value of signature "a" tag

   Status:  active

   Version:  1

6.3.2.  'header.s' for DKIM

   "header.s" for DKIM:

   Method:  dkim

   Definition:  [this document]

   ptype:  header

   property:  s

   Description:  value of signature "s" tag

   Status:  active

   Version:  1

6.4.  "Email Authentication Property Types" Registry

   [RFC7410] created the "Email Authentication Property Types" registry.

   No changes are made to the description of this registry.  However, it
   should be noted that Section 2.3 contains slightly different language
   than prior versions of this document, allowing a broader space from
   which to extract meaningful identifiers and report them through this

6.5.  "Email Authentication Result Names" Description

   No changes are made to the description of this registry.

6.6.  "Email Authentication Result Names" Update

   No changes are made to entries in this registry.

7.  Security Considerations

   The following security considerations apply when adding or processing
   the Authentication-Results header field:

7.1.  Forged Header Fields

   An MUA or filter that accesses a mailbox whose messages are handled
   by a non-conformant MTA, and understands Authentication-Results
   header fields, could potentially make false conclusions based on
   forged header fields.  A malicious user or agent could forge a header
   field using the DNS domain of a receiving ADMD as the authserv-id
   token in the value of the header field and, with the rest of the
   value, claim that the message was properly authenticated.  The non-
   conformant MTA would fail to strip the forged header field, and the
   MUA could inappropriately trust it.

   For this reason, it is best not to have processing of the
   Authentication-Results header field enabled by default; instead, it
   should be ignored, at least for the purposes of enacting filtering
   decisions, unless specifically enabled by the user or administrator
   after verifying that the border MTA is compliant.  It is acceptable
   to have an MUA aware of this specification but have an explicit list
   of hostnames whose Authentication-Results header fields are
   trustworthy; however, this list should initially be empty.

   Proposed alternative solutions to this problem were made some time
   ago and are listed below.  To date, they have not been developed due
   to lack of demand but are documented here should the information be
   useful at some point in the future:

   1.  Possibly the simplest is a digital signature protecting the
       header field, such as using [DKIM], that can be verified by an
       MUA by using a posted public key.  Although one of the main
       purposes of this document is to relieve the burden of doing
       message authentication work at the MUA, this only requires that
       the MUA learn a single authentication scheme even if a number of
       them are in use at the border MTA.  Note that [DKIM] requires
       that the From header field be signed, although in this
       application, the signing agent (a trusted MTA) likely cannot
       authenticate that value, so the fact that it is signed should be
       ignored.  Where the authserv-id is the ADMD's domain name, the
       authserv-id matching this valid internal signature's "d=" DKIM
       value is sufficient.

   2.  Another would be a means to interrogate the MTA that added the
       header field to see if it is actually providing any message
       authentication services and saw the message in question, but this
       isn't especially palatable given the work required to craft and
       implement such a scheme.

   3.  Yet another might be a method to interrogate the internal MTAs
       that apparently handled the message (based on Received header
       fields) to determine whether any of them conform to Section 5 of
       this memo.  This, too, has potentially high barriers to entry.

   4.  Extensions to [IMAP], [SMTP], and [POP3] could be defined to
       allow an MUA or filtering agent to acquire the authserv-id in use
       within an ADMD, thus allowing it to identify which
       Authentication-Results header fields it can trust.

   5.  On the presumption that internal MTAs are fully compliant with
       Section 3.6 of [MAIL] and the compliant internal MTAs are using
       their own hostnames or the ADMD's DNS domain name as the
       authserv-id token, the header field proposed here should always
       appear above a Received header added by a trusted MTA.  This can
       be used as a test for header field validity.

   Support for some of these is being considered for future work.

   In any case, a mechanism needs to exist for an MUA or filter to
   verify that the host that appears to have added the header field (a)
   actually did so and (b) is legitimately adding that header field for
   this delivery.  Given the variety of messaging environments deployed
   today, consensus appears to be that specifying a particular mechanism
   for doing so is not appropriate for this document.

   Mitigation of the forged header field attack can also be accomplished
   by moving the authentication results data into metadata associated
   with the message.  In particular, an [SMTP] extension could be
   established to communicate authentication results from the border MTA
   to intermediate and delivery MTAs; the latter of these could arrange
   to store the authentication results as metadata retrieved and
   rendered along with the message by an [IMAP] client aware of a
   similar extension in that protocol.  The delivery MTA would be told
   to trust data via this extension only from MTAs it trusts, and border
   MTAs would not accept data via this extension from any source.  There
   is no vector in such an arrangement for forgery of authentication
   data by an outside agent.

7.2.  Misleading Results

   Until some form of service for querying the reputation of a sending
   agent is widely deployed, the existence of this header field
   indicating a "pass" does not render the message trustworthy.  It is
   possible for an arriving piece of spam or other undesirable mail to
   pass checks by several of the methods enumerated above (e.g., a piece
   of spam signed using [DKIM] by the originator of the spam, which
   might be a spammer or a compromised system).  In particular, this
   issue is not resolved by forged header field removal discussed above.

   Hence, MUAs and downstream filters must take some care with use of
   this header even after possibly malicious headers are scrubbed.

7.3.  Header Field Position

   Despite the requirements of [MAIL], header fields can sometimes be
   reordered en route by intermediate MTAs.  The goal of requiring
   header field addition only at the top of a message is an
   acknowledgment that some MTAs do reorder header fields, but most do
   not.  Thus, in the general case, there will be some indication of
   which MTAs (if any) handled the message after the addition of the
   header field defined here.

7.4.  Reverse IP Query Denial-of-Service Attacks

   Section 4.6.4 of [SPF] describes a DNS-based denial-of-service attack
   for verifiers that attempt DNS-based identity verification of
   arriving client connections.  A verifier wishing to do this check and
   report this information needs to take care not to go to unbounded
   lengths to resolve "A" and "PTR" queries.  MUAs or other filters
   making use of an "iprev" result specified by this document need to be
   aware of the algorithm used by the verifier reporting the result and,
   especially, its limitations.

7.5.  Mitigation of Backscatter

   Failing to follow the instructions of Section 4.2 can result in a
   denial-of-service attack caused by the generation of [DSN] messages
   (or equivalent) to addresses that did not send the messages being

7.6.  Internal MTA Lists

   Section 5 describes a procedure for scrubbing header fields that may
   contain forged authentication results about a message.  A compliant
   installation will have to include, at each MTA, a list of other MTAs
   known to be compliant and trustworthy.  Failing to keep this list
   current as internal infrastructure changes may expose an ADMD to

7.7.  Attacks against Authentication Methods

   If an attack becomes known against an authentication method, clearly
   then the agent verifying that method can be fooled into thinking an
   inauthentic message is authentic, and thus the value of this header
   field can be misleading.  It follows that any attack against the
   authentication methods supported by this document is also a security
   consideration here.

7.8.  Intentionally Malformed Header Fields


   As with any other header field found in a message, it is possible for
   an attacker to add an Authentication-Results header field that is
   extraordinarily large or otherwise malformed in an attempt to
   discover or exploit weaknesses in header field parsing code.
   Implementers must thoroughly verify all such header fields received
   from MTAs and be robust against intentionally as well as
   unintentionally malformed header fields.

7.9.  Compromised Internal Hosts

   An internal MUA or MTA that has been compromised could generate mail
   with a forged From header field and a forged Authentication-Results
   header field that endorses it.  Although it is clearly a larger
   concern to have compromised internal machines than it is to prove the
   value of this header field, this risk can be mitigated by arranging
   that internal MTAs will remove this header field if it claims to have
   been added by a trusted border MTA (as described above), yet the
   [SMTP] connection is not coming from an internal machine known to be
   running an authorized MTA.  However, in such a configuration,
   legitimate MTAs will have to add this header field when legitimate
   internal-only messages are generated.  This is also covered in
   Section 5.

7.10.  Encapsulated Instances

   MIME messages can contain attachments of type "message/rfc822", which
   contain other messages.  Such an encapsulated message can also
   contain an Authentication-Results header field.  Although the
   processing of these is outside of the intended scope of this document
   (see Section 1.3), some early guidance to MUA developers is
   appropriate here.

   Since MTAs are unlikely to strip Authentication-Results header fields
   after mailbox delivery, MUAs are advised in Section 4.1 to ignore
   such instances within MIME attachments.  Moreover, when extracting a
   message digest to separate mail store messages or other media, such
   header fields should be removed so that they will never be
   interpreted improperly by MUAs that might later consume them.

7.11.  Reverse Mapping

   Although Section 3 of this memo includes explicit support for the
   "iprev" method, its value as an authentication mechanism is limited.
   Implementers of both this specification and agents that use the data
   it relays are encouraged to become familiar with the issues raised by
   [DNSOP-REVERSE] when deciding whether or not to include support for

8.  References

8.1.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
              RFC5234, January 2008,

   [DKIM]     Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,

              Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,

              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,

   [MAIL]     Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,

   [MIME]     Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,

   [RFC5451]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 5451, DOI 10.17487/
              RFC5451, April 2009,

   [RFC6008]  Kucherawy, M., "Authentication-Results Registration for
              Differentiating among Cryptographic Results", RFC 6008,
              DOI 10.17487/RFC6008, September 2010,

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
              February 2012, <>.

   [RFC6531]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,

   [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
              Email Headers", RFC 6532, DOI 10.17487/RFC6532,
              February 2012, <>.

   [RFC6577]  Kucherawy, M., "Authentication-Results Registration Update
              for Sender Policy Framework (SPF) Results", RFC 6577,
              DOI 10.17487/RFC6577, March 2012,

   [RFC7001]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 7001, DOI 10.17487/
              RFC7001, September 2013,

   [RFC7601]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 7601, DOI 10.17487/
              RFC7601, August 2015,

   [RFC8301]  Kitterman, S., "Cryptographic Algorithm and Key Usage
              Update to DomainKeys Identified Mail (DKIM)", RFC 8301,
              DOI 10.17487/RFC8301, January 2018,

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,

8.2.  Informative References

   [ADSP]     Allman, E., Fenton, J., Delany, M., and J. Levine,
              "DomainKeys Identified Mail (DKIM) Author Domain Signing
              Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617,
              August 2009, <>.

   [AR-VBR]   Kucherawy, M., "Authentication-Results Registration for
              Vouch by Reference Results", RFC 6212, DOI 10.17487/
              RFC6212, April 2011,

   [ATPS]     Kucherawy, M., "DomainKeys Identified Mail (DKIM)
              Authorized Third-Party Signatures", RFC 6541,
              DOI 10.17487/RFC6541, February 2012,

   [AUTH]     Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
              Extension for Authentication", RFC 4954, DOI 10.17487/
              RFC4954, July 2007,

              Kucherawy, M., "Email Authentication Status Codes",
              RFC 7372, DOI 10.17487/RFC7372, September 2014,

   [DMARC]    Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [DNS-IP6]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", RFC 3596,
              DOI 10.17487/RFC3596, October 2003,

              Senie, D. and A. Sullivan, "Considerations for the use of
              DNS Reverse Mapping", Work in Progress, draft-ietf-dnsop-
              reverse-mapping-considerations-06, March 2008.

              Delany, M., "Domain-Based Email Authentication Using
              Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
              DOI 10.17487/RFC4870, May 2007,

   [DSN]      Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              DOI 10.17487/RFC3464, January 2003,

              Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009,

              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,

   [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,

   [PRA]      Lyon, J., "Purported Responsible Address in E-Mail
              Messages", RFC 4407, DOI 10.17487/RFC4407, April 2006,

   [RFC7410]  Kucherawy, M., "A Property Types Registry for the
              Authentication-Results Header Field", RFC 7410,
              DOI 10.17487/RFC7410, December 2014,

   [RRVS]     Mills, W. and M. Kucherawy, "The Require-Recipient-Valid-
              Since Header Field and SMTP Service Extension", RFC 7293,
              DOI 10.17487/RFC7293, July 2014,

              Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,

              Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
              RFC 4406, DOI 10.17487/RFC4406, April 2006,

              Melnikov, A., "Authentication-Results Registration for
              S/MIME Signature Verification", RFC 7281, DOI 10.17487/
              RFC7281, June 2014,

   [SPF]      Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,

   [VBR]      Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
              Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009,

Appendix A.  Legacy MUAs

   Implementers of this protocol should be aware that many MUAs are
   unlikely to be retrofitted to support the new header field and its
   semantics.  In the interests of convenience and quicker adoption, a
   delivery MTA might want to consider adding things that are processed
   by existing MUAs in addition to the Authentication-Results header
   field.  One suggestion is to include a Priority header field, on
   messages that don't already have such a header field, containing a
   value that reflects the strength of the authentication that was
   accomplished, e.g., "low" for weak or no authentication, "normal" or
   "high" for good or strong authentication.

   Some modern MUAs can already filter based on the content of this
   header field.  However, there is keen interest in having MUAs make
   some kind of graphical representation of this header field's meaning
   to end users.  Until this capability is added (i.e., while this
   specification and its successors continue to be adopted), other
   interim means of conveying authentication results may be necessary.

Appendix B.  Authentication-Results Examples

   This section presents some examples of the use of this header field
   to indicate authentication results.

B.1.  Trivial Case; Header Field Not Present

   The trivial case:

        Received: from
                      ( [])
                  by (8.11.6/8.11.6)
                      with ESMTP id g1G0r1kA003489;
                  Fri, Feb 15 2002 17:19:07 -0800
        Date: Fri, Feb 15 2002 16:54:30 -0800
        Message-Id: <>
        Subject: here's a sample

        Hello!  Goodbye!

   Example 1: Trivial Case

   The Authentication-Results header field is completely absent.  The
   MUA may make no conclusion about the validity of the message.  This
   could be the case because the message authentication services were
   not available at the time of delivery, or no service is provided, or
   the MTA is not in compliance with this specification.

B.2.  Nearly Trivial Case; Service Provided, but No Authentication Done

   A message that was delivered by an MTA that conforms to this
   specification but provides no actual message authentication service:

        Authentication-Results: 1; none
        Received: from
                      ( [])
                  by (8.11.6/8.11.6)
                      with ESMTP id g1G0r1kA003489;
                  Fri, Feb 15 2002 17:19:07 -0800
        Date: Fri, Feb 15 2002 16:54:30 -0800
        Message-Id: <>
        Subject: here's a sample

        Hello!  Goodbye!

   Example 2: Header Present but No Authentication Done

   The Authentication-Results header field is present, showing that the
   delivering MTA conforms to this specification.  It used its DNS
   domain name as the authserv-id.  The presence of "none" (and the
   absence of any method or result tokens) indicates that no message
   authentication was done.  The version number of the specification to
   which the field's content conforms is explicitly provided.

B.3.  Service Provided, Authentication Done

   A message that was delivered by an MTA that conforms to this
   specification and applied some message authentication:

        Received: from
                      ( [])
                  by (8.11.6/8.11.6)
                      with ESMTP id g1G0r1kA003489;
                  Fri, Feb 15 2002 17:19:07 -0800
        Date: Fri, Feb 15 2002 16:54:30 -0800
        Message-Id: <>
        Subject: here's a sample

        Hello!  Goodbye!

   Example 3: Header Reporting Results

   The Authentication-Results header field is present, indicating that
   the border MTA conforms to this specification.  The authserv-id is
   once again the DNS domain name.  Furthermore, the message was
   authenticated by that MTA via the method specified in [SPF].  Note
   that since that method cannot authenticate the local-part, it has
   been omitted from the result's value.  The MUA could extract and
   relay this extra information if desired.

B.4.  Service Provided, Several Authentications Done, Single MTA

   A message that was relayed inbound via a single MTA that conforms to
   this specification and applied three different message authentication

                  auth=pass (cram-md5);
        Received: from (8.11.6/8.11.6)
                      ( [])
                  by (8.11.6/8.11.6)
                      with ESMTPA id g1G0r1kA003489;
                  Fri, Feb 15 2002 17:19:07 -0800
        Date: Fri, Feb 15 2002 16:54:30 -0800
        Message-Id: <>
        Subject: here's a sample

        Hello!  Goodbye!

   Example 4: Headers Reporting Results from One MTA

   The Authentication-Results header field is present, indicating that
   the delivering MTA conforms to this specification.  Once again, the
   receiving DNS domain name is used as the authserv-id.  Furthermore,
   the sender authenticated herself/himself to the MTA via a method
   specified in [AUTH], and both SPF and Sender ID checks were done and
   passed.  The MUA could extract and relay this extra information if

   Two Authentication-Results header fields are not required since the
   same host did all of the checking.  The authenticating agent could
   have consolidated all the results into one header field.

   This example illustrates a scenario in which a remote user on a
   dial-up connection ( sends mail to a border MTA
   ( using SMTP authentication to prove identity.  The
   dial-up provider has been explicitly authorized to relay mail as, producing "pass" results from the SPF and Sender ID

B.5.  Service Provided, Several Authentications Done, Different MTAs

   A message that was relayed inbound by two different MTAs that conform
   to this specification and applied multiple message authentication

                  dkim=pass (good signature)
        Received: from
                      ( [])
                  by (8.11.6/8.11.6)
                      with ESMTP id i7PK0sH7021929;
                  Fri, Feb 15 2002 17:19:22 -0800
        DKIM-Signature:  v=1; a=rsa-sha256; s=gatsby;;
                  t=1188964191; c=simple/simple; h=From:Date:To:Subject:
                  b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
                  auth=pass (cram-md5);
        Received: from
                      ( [])
                  by (8.11.6/8.11.6)
                      with ESMTPA id g1G0r1kA003489;
                  Fri, Feb 15 2002 17:19:07 -0800
        Date: Fri, Feb 15 2002 16:54:30 -0800
        Message-Id: <>
        Subject: here's a sample

        Hello!  Goodbye!

   Example 5: Headers Reporting Results from Multiple MTAs

   The Authentication-Results header field is present, indicating
   conformance to this specification.  Once again, the authserv-id used
   is the recipient's DNS domain name.  The header field is present
   twice because two different MTAs in the chain of delivery did
   authentication tests.  The first MTA,,
   reports that SMTP AUTH and SPF were both used and that the former
   passed while the latter failed.  In the SMTP AUTH case, additional
   information is provided in the comment field, which the MUA can
   choose to render if desired.

   The second MTA,, reports that it did a
   Sender ID test (which failed) and a DKIM test (which passed).  Again,
   additional data about one of the tests is provided as a comment,
   which the MUA may choose to render.  Also noteworthy here is the fact
   that there is a DKIM signature added by that assured the
   integrity of the lower Authentication-Results field.

   Since different hosts did the two sets of authentication checks, the
   header fields cannot be consolidated in this example.

   This example illustrates more typical transmission of mail into from a user on a dial-up connection  The
   user appears to be legitimate as he/she had a valid password allowing
   authentication at the border MTA using SMTP AUTH.  The SPF test
   failed since has not granted's dial-up
   network authority to relay mail on its behalf.  The Sender ID test
   failed since has not granted
   authority to relay mail on its behalf.  However, the DKIM test passed
   because the sending user had a private key matching one of's published public keys and used
   it to sign the message.

B.6.  Service Provided, Multi-tiered Authentication Done

   A message that had authentication done at various stages, one of
   which was outside the receiving ADMD:

              dkim=pass reason="good signature"
              dkim=fail reason="bad signature"
        Received: from
                  ( [])
              by (8.11.6/8.11.6)
                  for <>
                  with ESMTP id i7PK0sH7021929;
              Fri, Feb 15 2002 17:19:22 -0800
        DKIM-Signature: v=1; a=rsa-sha256; s=furble;
    ; t=1188964198; c=relaxed/simple;
              b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3=
              dkim=pass (good signature)
        Received: from
                  ( [])
              by (8.11.6/8.11.6)
                  with ESMTP id g1G0r1kA003489;
              Fri, Feb 15 2002 17:19:07 -0800
        DKIM-Signature: v=1; a=rsa-sha256; s=gatsby;
              t=1188964191; c=simple/simple;
              b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
        Date: Fri, Feb 15 2002 16:54:30 -0800
        Message-Id: <>
        Subject: here's a sample

   Example 6: Headers Reporting Results from Multiple MTAs in Different

   In this example, we see multi-tiered authentication with an extended
   trust boundary.

   The message was sent from someone at's New York office
   ( to a mailing list managed at an intermediary.

   The message was signed at the origin using DKIM.

   The message was sent to a mailing list service provider called, which is used by  There, is expanded to a long list of recipients, one of
   whom is at the Chicago office.  In this example, we will assume that
   the trust boundary for includes the mailing list
   server at

   The mailing list server there first authenticated the message and
   affixed an Authentication-Results header field indicating such using
   its DNS domain name for the authserv-id.  It then altered the message
   by affixing some footer text to the body, including some
   administrivia such as unsubscription instructions.  Finally, the
   mailing list server affixes a second DKIM signature and begins
   distribution of the message.

   The border MTA for explicitly trusts results from, so that header field is not removed.  It
   performs evaluation of both signatures and determines that the first
   (most recent) is a "pass" but, because of the aforementioned
   modifications, the second is a "fail".  However, the first signature
   included the Authentication-Results header added at mail- that validated the second signature.  Thus,
   indirectly, it can be determined that the authentications claimed by
   both signatures are indeed valid.

   Note that two styles of presenting metadata about the result are in
   use here.  In one case, the "reason=" clause is present, which is
   intended for easy extraction by parsers; in the other case, the CFWS
   production of the ABNF is used to include such data as a header field
   comment.  The latter can be harder for parsers to extract given the
   varied supported syntaxes of mail header fields.

B.7.  Comment-Heavy Example

   The formal syntax permits comments within the content in a number of
   places.  For the sake of illustration, this example is also legal:

       Authentication-Results: (foobar) 1 (baz);
           dkim (Because I like it) / 1 (One yay) = (wait for it) fail
             policy (A dot can go here) . (like that) expired
             (this surprised me) = (as I wasn't expecting it) 1362471462

   Example 7: A Very Comment-Heavy but Perfectly Legal Example

Appendix C.  Operational Considerations about Message Authentication

   This protocol is predicated on the idea that authentication (and
   presumably in the future, reputation) work is typically done by
   border MTAs rather than MUAs or intermediate MTAs; the latter merely
   make use of the results determined by the former.  Certainly this is
   not mandatory for participation in electronic mail or message
   authentication, but this protocol and its deployment to date are
   based on that model.  The assumption satisfies several common ADMD

   1.  Service operators prefer to resolve the handling of problem
       messages as close to the border of the ADMD as possible.  This
       enables, for example, rejection of messages at the SMTP level
       rather than generating a DSN internally.  Thus, doing any of the
       authentication or reputation work exclusively at the MUA or
       intermediate MTA renders this desire unattainable.

   2.  Border MTAs are more likely to have direct access to external
       sources of authentication or reputation information since modern
       MUAs are more likely to be heavily firewalled.  Thus, some MUAs
       might not even be able to complete the task of performing
       authentication or reputation evaluations without complex proxy
       configurations or similar burdens.

   3.  MUAs rely upon the upstream MTAs within their trust boundaries to
       make correct (as much as is possible) evaluations about the
       message's envelope, header, and content.  Thus, MUAs don't need
       to know how to do the work that upstream MTAs do; they only need
       the results of that work.

   4.  Evaluations about the quality of a message, from simple token
       matching (e.g., a list of preferred DNS domains) to cryptanalysis
       (e.g., public/private key work), do have a cost and thus need to
       be minimized.  To that end, performing those tests at the border
       MTA is far preferred to doing that work at each MUA that handles
       a message.  If an ADMD's environment adheres to common messaging
       protocols, a reputation query or an authentication check
       performed by a border MTA would return the same result as the
       same query performed by an MUA.  By contrast, in an environment
       where the MUA does the work, a message arriving for multiple
       recipients would thus cause authentication or reputation
       evaluation to be done more than once for the same message (i.e.,
       at each MUA), causing needless amplification of resource use and
       creating a possible denial-of-service attack vector.

   5.  Minimizing change is good.  As new authentication and reputation
       methods emerge, the list of methods supported by this header
       field would presumably be extended.  If MUAs simply consume the
       contents of this header field rather than actually attempt to do
       authentication and/or reputation work, then MUAs only need to
       learn to parse this header field once; emergence of new methods
       requires only a configuration change at the MUAs and software
       changes at the MTAs (which are presumably fewer in number).  When
       choosing to implement these functions in MTAs vs. MUAs, the
       issues of individual flexibility, infrastructure inertia, and
       scale of effort must be considered.  It is typically easier to
       change a single MUA than an MTA because the modification affects
       fewer users and can be pursued with less care.  However, changing
       many MUAs is more effort than changing a smaller number of MTAs.

   6.  For decisions affecting message delivery and display, assessment
       based on authentication and reputation is best performed close to
       the time of message transit, as a message makes its journey
       toward a user's inbox, not afterwards.  DKIM keys and IP address
       reputations, etc., can change over time or even become invalid,
       and users can take a long time to read a message once delivered.
       The value of this work thus degrades, perhaps quickly, once the
       delivery process has completed.  This seriously diminishes the
       value of this work when done elsewhere than at MTAs.

   Many operational choices are possible within an ADMD, including the
   venue for performing authentication and/or reputation assessment.
   The current specification does not dictate any of those choices.
   Rather, it facilitates those cases in which information produced by
   one stage of analysis needs to be transported with the message to the
   next stage.

Appendix D.  Changes Since RFC7601

   o  Added IANA registration for DKIM "a" and "s" properties.

   o  Include EAI guidance.

   o  Adjust some ABNF tokens and names for easier inclusion by other

Appendix E.  Acknowledgments

   The author wishes to acknowledge the following individuals for their
   review and constructive criticism of this document: Seth Blank, Tim
   Draegen, John Levine, Scott Kitterman, and Alessandro Vesely.

Author's Address

   Murray S. Kucherawy
   270 Upland Drive
   San Francisco, CA  94127
   United States