Internet Engineering Task Force (IETF) M. Kucherawy
Request for Comments: 7601 August 2015
Obsoletes: 7001, 7410
Category: Standards Track
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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
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.
This specification is not intended to be restricted to domain-based
authentication schemes, but the existing schemes in that family have
proven to be a good starting point for implementations. The goal 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.
The header field defined in this document is expected to serve
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 (supposedly)
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.
This document establishes no new requirements on existing protocols
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
This section defines various terms used throughout this document.
1.5.1. Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
"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
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 proposal groups them both into a single header field.
1.5.3. 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
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.
1.5.4. 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 are present
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 more 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 = "Authentication-Results:" [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 "ptype.property" construction
"local-part" is defined in Section 3.4.1 of [MAIL], and "CFWS" is
defined in Section 3.2.2 of [MAIL].
"Keyword" is defined in Section 4.1.2 of [SMTP].
The "value" is as defined in Section 5.1 of [MIME].
The "domain-name" is as defined in Section 3.5 of [DKIM].
The "Keyword" used in "result" above is further constrained by the
necessity of being enumerated in Section 2.7.
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 has a result to
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
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 which particular part of the message the
reported data were extracted.
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
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
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
... dkim=fail 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.
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; these 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
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
Examples of valid authentication identifiers are "example.com",
"mail.example.org", "ms1.newyork.example.com", 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
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
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".
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".)
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.
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
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
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.
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 "email@example.com":
MAIL FROM:<firstname.lastname@example.org> AUTH=<email@example.com>
This could result in a "resinfo" construction like so:
; auth=pass firstname.lastname@example.org email@example.com
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").
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.
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.