Domain-based Message Authentication, Reporting, and Conformance (DMARC)

2.1. High-Level Goals

DMARC has the following high-level goals:¶

• Allow Domain Owners and PSOs to assert their severity of concern for authentication failures for messages purporting to have authorship within the domain.¶

• Allow Domain Owners and PSOs to verify their authentication deployment.¶

• Minimize implementation complexity for both senders and receivers, as well as the impact on handling and delivery of legitimate messages.¶

• Reduce the amount of successfully delivered spoofed email.¶

• Work at Internet scale.¶

2.2. Out of Scope

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

• different treatment of messages that are not authenticated versus those that fail authentication;¶

• evaluation of anything other than RFC5322.From header field;¶

• multiple reporting formats;¶

• publishing policy other than via the DNS;¶

• reporting or otherwise evaluating other than the last-hop IP address;¶

• attacks in the From: header field, also known as "display name" attacks;¶

• authentication of entities other than domains, since DMARC is built upon SPF and DKIM, which authenticate domains; and¶

• content analysis.¶

2.3. Scalability

Scalability is a major issue for systems that need to operate in a system as widely deployed as current SMTP email. For this reason, DMARC seeks to avoid the need for third parties or pre-sending agreements between senders and receivers. This preserves the positive aspects of the current email infrastructure.¶

Although DMARC does not introduce third-party senders (namely external agents authorized to send on behalf of an operator) to the email-handling flow, it also does not preclude them. Such third parties are free to provide services in conjunction with DMARC.¶

2.4. Anti-Phishing

DMARC is designed to prevent bad actors from sending mail that claims to come from legitimate senders, particularly senders of transactional email (official mail that is about business transactions). One of the primary uses of this kind of spoofed mail is phishing (enticing users to provide information by pretending to be the legitimate service requesting the information). Thus, DMARC is significantly informed by ongoing efforts to enact large-scale, Internet-wide anti-phishing measures.¶

Although DMARC can only be used to combat specific forms of exact- domain spoofing directly, the DMARC mechanism has been found to be useful in the creation of reliable and defensible message streams.¶

DMARC does not attempt to solve all problems with spoofed or otherwise fraudulent email. In particular, it does not address the use of visually similar domain names ("cousin domains") or abuse of the RFC5322.From human-readable <display-name>.¶

3.1. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

Readers are encouraged to be familiar with the contents of [RFC5598]. In particular, that document defines various roles in the messaging infrastructure that can appear the same or separate in various contexts. For example, a Domain Owner could, via the messaging security mechanisms on which DMARC is based, delegate the ability to send mail as the Domain Owner to a third party with another role. This document does not address the distinctions among such roles; the reader is encouraged to become familiar with that material before continuing.¶

3.2. Authenticated Identifiers

Domain-level identifiers that are validated using authentication technologies are referred to as "Authenticated Identifiers". See Section 4.1 for details about the supported mechanisms.¶

3.3. Author Domain

The domain name of the apparent author, as extracted from the From: header field.¶

3.4. Domain Owner

An entity or organization that owns a DNS domain. The term "owns" here indicates that the entity or organization being referenced holds the registration of that DNS domain. Domain Owners range from complex, globally distributed organizations, to service providers working on behalf of non-technical clients, to individuals responsible for maintaining personal domains. This specification uses this term as analogous to an Administrative Management Domain as defined in [RFC5598]. It can also refer to delegates, such as Report Receivers, when those are outside of their immediate management domain.¶

3.5. Identifier Alignment

When the domain in the address in the From: header field has the same Organizational Domain as a domain validated by SPF or DKIM (or both), it has Identifier Alignment. (see below)¶

3.6. Longest PSD

The term Longest PSD is defined in [DMARC-PSD].¶

3.7. Mail Receiver

The entity or organization that receives and processes email.
Mail Receivers operate one or more Internet- facing Mail Transport Agents (MTAs).¶

3.8. Non-existent Domains

For DMARC purposes, a non-existent domain is a domain for which there is an NXDOMAIN or NODATA response for A, AAAA, and MX records. This is a broader definition than that in [RFC8020].¶

3.9. Organizational Domain

The domain that was registered with a domain name registrar. In the absence of more accurate methods, heuristics are used to determine this, since it is not always the case that the registered domain name is simply a top-level DNS domain plus one component (e.g., "example.com", where "com" is a top-level domain). The Organizational Domain is determined by applying the algorithm found in Section 3.15.¶

3.10. Public Suffix Domain (PSD)

The term Public Suffix Domain is defined in [DMARC-PSD].¶

3.11. Public Suffix Operator (PSO)

The term Public Suffix Operator is defined in [DMARC-PSD].¶

3.12. PSO Controlled Domain Names

The term PSO Controlled Domain Names is defined in [DMARC-PSD].¶

3.13. Report Receiver

An operator that receives reports from another operator implementing the reporting mechanisms described in this document. Such an operator might be receiving reports about messages related to a domain for which it is the Domain Owner or PSO, or reports about messages related to another operator's domain. This term applies collectively to the system components that receive and process these reports and the organizations that operate them.¶

3.14. More on Identifier Alignment

Email authentication technologies authenticate various (and disparate) aspects of an individual message. For example, DKIM [RFC6376] authenticates the domain that affixed a signature to the message, while SPF [RFC7208] can authenticate either the domain that appears in the RFC5321.MailFrom (MAIL FROM) portion of [RFC5322] or the RFC5321.EHLO/ HELO domain, or both. These may be different domains, and they are typically not visible to the end user.¶

DMARC authenticates use of the RFC5322.From domain by requiring that it have the same Organizational Domain (be aligned with) as an Authenticated Identifier. The RFC5322.From domain was selected as the central identity of the DMARC mechanism because it is a required message header field and therefore guaranteed to be present in compliant messages, and most Mail User Agents (MUAs) represent the RFC5322.From header field as the originator of the message and render some or all of this header field's content to end users.¶

Thus, this field is the one used by end users to identify the source of the message and therefore is a prime target for abuse. Many high-profile email sources, such as email service providers, require that the sending agent have authenticated before email can be generated. Thus, for these mailboxes, the mechanism described in this document provides recipient end users with strong evidence that the message was indeed originated by the agent they associate with that mailbox, if the end user knows that these various protections have been provided.¶

Domain names in this context are to be compared in a case-insensitive manner, per [RFC4343].¶

It is important to note that Identifier Alignment cannot occur with a message that is not valid per [RFC5322], particularly one with a malformed, absent, or repeated RFC5322.From header field, since in that case there is no reliable way to determine a DMARC policy that applies to the message. Accordingly, DMARC operation is predicated on the input being a valid RFC5322 message object, and handling of such non-compliant cases is outside of the scope of this specification. Further discussion of this can be found in Section 6.7.1.¶

Each of the underlying authentication technologies that DMARC takes as input yields authenticated domains as their outputs when they succeed.¶

3.15. Determining The Organizational Domain

The Organizational Domain is determined using the following algorithm:¶

1. Acquire a "public suffix" list, i.e., a list of DNS domain names reserved for registrations. Some country Top-Level Domains (TLDs) make specific registration requirements, e.g., the United Kingdom places company registrations under ".co.uk"; other TLDs such as ".com" appear in the IANA registry of top-level DNS domains. A public suffix list is the union of all of these. Appendix A.6.1 contains some discussion about obtaining a public suffix list.¶

2. Break the subject DNS domain name into a set of "n" ordered labels. Number these labels from right to left; e.g., for "example.com", "com" would be label 1 and "example" would be label 2.¶

3. Search the public suffix list for the name that matches the largest number of labels found in the subject DNS domain. Let that number be "x".¶

4. Construct a new DNS domain name using the name that matched from the public suffix list and prefixing to it the "x+1"th label from the subject domain. This new name is the Organizational Domain.¶

Thus, since "com" is an IANA-registered TLD, a subject domain of "a.b.c.d.example.com" would have an Organizational Domain of "example.com".¶

The process of determining a suffix is currently a heuristic one. No list is guaranteed to be accurate or current.¶

Ticket 109, Original text: (Seems like these two paragraphs should be moved elsewhere?)¶

In addition to Mediators, mail that is sent by authorized,
independent third parties might not be sent with Identifier
Alignment, also preventing a "pass" result.

Issues specific to the use of policy mechanisms alongside DKIM are
further discussed in [@RFC6377], particularly Section 5.2.

4.1. Authentication Mechanisms

The following mechanisms for determining Authenticated Identifiers are supported in this version of DMARC:¶

• DKIM, [RFC6376], which provides a domain-level identifier in the content of the "d=" tag of a validated DKIM-Signature header field.¶

• SPF, [RFC7208], which can authenticate both the domain found in an [RFC5322] HELO/EHLO command (the HELO identity) and the domain found in an SMTP MAIL command (the MAIL FROM identity). Section 2.4 of [RFC7208] describes MAIL FROM processing for cases in which the MAIL command has a null path.¶

4.2. Key Concepts

DMARC policies are published by the Domain Owner or PSO, and retrieved by the Mail Receiver during the SMTP session, via the DNS.¶

DMARC's filtering function is based on whether the RFC5322.From domain is aligned with (has the same Organizational Domain as) an authenticated domain name from SPF or DKIM. When a DMARC policy is published for the domain name found in the RFC5322.From header field, and that domain name is not validated through SPF or DKIM, the handling of that message can be affected by that DMARC policy when delivered to a participating receiver.¶

It is important to note that the authentication mechanisms employed by DMARC authenticate only a DNS domain and do not authenticate the local-part of any email address identifier found in a message, nor do they validate the legitimacy of message content.¶

DMARC's feedback component involves the collection of information about received messages claiming to be from the Author Domain for periodic aggregate reports to the Domain Owner or PSO. The parameters and format for such reports are discussed in [DMARC-Aggregate-Reporting]¶

A DMARC-enabled Mail Receiver might also generate per-message reports that contain information related to individual messages that fail SPF and/or DKIM. Per-message failure reports are a useful source of information when debugging deployments (if messages can be determined to be legitimate even though failing authentication) or in analyzing attacks. The capability for such services is enabled by DMARC but defined in other referenced material such as [RFC6591] and [DMARC-Failure-Reporting]¶

A message satisfies the DMARC checks if at least one of the supported authentication mechanisms:¶

1. produces a "pass" result, and¶

2. produces that result based on an identifier that is in alignment, as defined in Section 3.¶

4.3. Flow Diagram

 +---------------+                             +--------------------+
 | Author Domain |< . . . . . . . . . . . .    | Return-Path Domain |
 +---------------+                        .    +--------------------+
     |                                    .               ^
     V                                    V               .
 +-----------+     +--------+       +----------+          v
 |   MSA     |<***>|  DKIM  |       |   DMARC  |     +----------+
 |  Service  |     | Signer |       | Verifier |<***>|    SPF   |
 +-----------+     +--------+       +----------+  *  | Verifier |
     |                                    ^       *  +----------+
     |                                    *       *
     V                                    v       *
  +------+        (~~~~~~~~~~~~)      +------+    *  +----------+
  | sMTA |------->( other MTAs )----->| rMTA |    **>|   DKIM   |
  +------+        (~~~~~~~~~~~~)      +------+       | Verifier |
                                         |           +----------+
                                         |                ^
                                         V                .
                                  +-----------+           .
                    +---------+   |    MDA    |           v
                    |  User   |<--| Filtering |      +-----------+
                    | Mailbox |   |  Engine   |      |   DKIM    |
                    +---------+   +-----------+      |  Signing  |
                                                     | Domain(s) |
                                                     +-----------+

  MSA = Mail Submission Agent
  MDA = Mail Delivery Agent

The above diagram shows a simple flow of messages through a DMARC- aware system. Solid lines denote the actual message flow, dotted lines involve DNS queries used to retrieve message policy related to the supported message authentication schemes, and asterisk lines indicate data exchange between message-handling modules and message authentication modules. "sMTA" is the sending MTA, and "rMTA" is the receiving MTA.¶

Put simply, when a message reaches a DMARC-aware rMTA, a DNS query will be initiated to determine if the author domain has published a DMARC policy. If a policy is found, the rMTA will use the results of SPF and DKIM validation checks to determine the ultimate DMARC authentication status. The DMARC status can then factor into the message handling decision made by the recipient's mail sytsem.¶

More details on specific actions for the parties involved can be found in Section 6.5 and Section 6.7.¶

6.1. DMARC Policy Record

Domain Owner and PSO DMARC preferences are stored as DNS TXT records in subdomains named "_dmarc". For example, the Domain Owner of "example.com" would post DMARC preferences in a TXT record at "_dmarc.example.com". Similarly, a Mail Receiver wishing to query for DMARC preferences regarding mail with an RFC5322.From domain of "example.com" would issue a TXT query to the DNS for the subdomain of "_dmarc.example.com". The DNS-located DMARC preference data will hereafter be called the "DMARC record".¶

DMARC's use of the Domain Name Service is driven by DMARC's use of domain names and the nature of the query it performs. The query requirement matches with the DNS, for obtaining simple parametric information. It uses an established method of storing the information, associated with the target domain name, namely an isolated TXT record that is restricted to the DMARC context. Use of the DNS as the query service has the benefit of reusing an extremely well-established operations, administration, and management infrastructure, rather than creating a new one.¶

Per [RFC1035], a TXT record can comprise several "character-string" objects. Where this is the case, the module performing DMARC evaluation MUST concatenate these strings by joining together the objects in order and parsing the result as a single string.¶

6.2. DMARC URIs

[RFC3986] defines a generic syntax for identifying a resource. The DMARC mechanism uses this as the format by which a Domain Owner or PSO specifies the destination for the two report types that are supported.¶

The place such URIs are specified (see Section 6.3) allows a list of these to be provided. The list of URIs is separated by commas (ASCII 0x2c). A report SHOULD be sent to each listed URI provided in the DMARC record.¶

A formal definition is provided in Section 6.4.¶

6.3. General Record Format

DMARC records follow the extensible "tag-value" syntax for DNS-based key records defined in DKIM [RFC6376].¶

Section 10 creates a registry for known DMARC tags and registers the initial set defined in this document. Only tags defined in this document or in later extensions, and thus added to that registry, are to be processed; unknown tags MUST be ignored.¶

The following tags are introduced as the initial valid DMARC tags:¶

adkim:

(plain-text; OPTIONAL; default is "r".) Indicates whether strict or relaxed DKIM Identifier Alignment mode is required by the Domain Owner. See Section 3.14.1 for details. Valid values are as follows:¶

r:

relaxed mode¶

s:

strict mode¶

aspf:

(plain-text; OPTIONAL; default is "r".) Indicates whether strict or relaxed SPF Identifier Alignment mode is required by the Domain Owner. See Section 3.14.2 for details. Valid values are as follows:¶

r:

relaxed mode¶

s:

strict mode¶

fo:

Failure reporting options (plain-text; OPTIONAL; default is "0") Provides requested options for generation of failure reports. Report generators MAY choose to adhere to the requested options. This tag's content MUST be ignored if a "ruf" tag (below) is not also specified. Failure reporting options are shown below. The value of this tag is either "0", "1", or a colon-separated list of the options represented by alphabetic characters.¶

0:

Generate a DMARC failure report if all underlying authentication mechanisms fail to produce an aligned "pass" result.¶

1:

Generate a DMARC failure report if any underlying authentication mechanism produced something other than an aligned "pass" result.¶

d:

Generate a DKIM failure report if the message had a signature that failed evaluation, regardless of its alignment. DKIM- specific reporting is described in [RFC6651].¶

s:

Generate an SPF failure report if the message failed SPF evaluation, regardless of its alignment. SPF-specific reporting is described in [RFC6652].¶

np:

Domain Owner Assessment Policy for non-existent subdomains (plain-text; OPTIONAL). Indicates the severity of concern the Domain Owner or PSO has for mail using non-existent subdomains of the domain queried. It applies only to non-existent subdomains of the domain queried and not to either existing subdomains or the domain itself. Its syntax is identical to that of the "p" tag defined below. If the "np" tag is absent, the policy specified by the "sp" tag (if the "sp" tag is present) or the policy specified by the "p" tag, if the "sp" tag is not present, MUST be applied for non-existent subdomains. Note that "np" will be ignored for DMARC records published on subdomains of Organizational Domains and PSDs due to the effect of the DMARC policy discovery mechanism described in Section 6.7.3.¶

p:

Domain Owner Assessment Policy (plain-text; RECOMMENDED for policy records). Indicates the severity of concern the Domain Owner or PSO has for mail using its domain but not passing DMARC validation. Policy applies to the domain queried and to subdomains, unless subdomain policy is explicitly described using the "sp" or "np" tags. This tag is mandatory for policy records only, but not for third-party reporting records (see [DMARC-Aggregate-Reporting] and [DMARC-Failure-Reporting]) Possible values are as follows:¶

none:

The Domain Owner offers no expression of concern.¶

quarantine:

The Domain Owner considers such mail to be suspicious. It is possible the mail is valid, although the failure creates a significant concern.¶

reject:

The Domain Owner considers all such failures to be a clear indication that the use of the domain name is not valid. See Section 9.3 for some discussion of SMTP rejection methods and their implications.¶

pct:

(plain-text integer between 0 and 100, inclusive; OPTIONAL; default is 100). For the RFC5322.From domain to which the DMARC record applies, the "pct" tag is the percentage of messages producing a DMARC result of "fail" to which the Domain Owner wishes its preferred handling policy be applied. However, this MUST NOT be applied to the DMARC-generated reports, all of which must be sent and received unhindered.¶

The prospect of "all or nothing" is recognized as preventing many organizations from experimenting with strong authentication-based mechanisms. The intent of the "pct" tag is to give Domain Owners a method to enact a slow rollout of enforcement of the DMARC mechanism, but as is discussed in Section 6.7.4 the tag is at best a rough approximation of such a method.¶

rua:

Addresses to which aggregate feedback is to be sent (comma- separated plain-text list of DMARC URIs; OPTIONAL). [DMARC-Aggregate-Reporting] discusses considerations that apply when the domain name of a URI differs from that of the domain advertising the policy. See Section 11.5 for additional considerations. Any valid URI can be specified.
A Mail Receiver MUST implement support for a "mailto:" URI, i.e., the ability to send a DMARC report via electronic mail. If not provided, Mail Receivers MUST NOT generate aggregate feedback reports. URIs not supported by Mail Receivers MUST be ignored. The aggregate feedback report format is described in the DMARC reporting documents.¶

ruf:

Addresses to which message-specific failure information is to be reported (comma-separated plain-text list of DMARC URIs; OPTIONAL). If present, the Domain Owner or PSO is requesting Mail Receivers to send detailed failure reports about messages that fail the DMARC evaluation in specific ways (see the "fo" tag above). The format of the message to be generated MUST follow the format specified for the "rf" tag. [DMARC-Failure-Reporting] discusses considerations that apply when the domain name of a URI differs from that of the domain advertising the policy. A Mail Receiver MUST implement support for a "mailto:" URI, i.e., the ability to send a DMARC report via electronic mail. If not provided, Mail Receivers MUST NOT generate failure reports. See Section 11.5 for additional considerations.¶

sp:

Domain Owner Assessment Policy for all subdomains (plain-text; OPTIONAL). Indicates the severity of concern the Domain Owner or PSO has for mail using an existing subdomain of the domain queried but not passing DMARC validation. It applies only to subdomains of the domain queried and not to the domain itself. Its syntax is identical to that of the "p" tag defined above. If both the "sp" tag is absent and the "np" tag is either absent or not applicable, the policy specified by the "p" tag MUST be applied for subdomains. Note that "sp" will be ignored for DMARC records published on subdomains of Organizational Domains due to the effect of the DMARC policy discovery mechanism described in Section 6.7.3.¶

v:

Version (plain-text; REQUIRED). Identifies the record retrieved as a DMARC record. It MUST have the value of "DMARC1". The value of this tag MUST match precisely; if it does not or it is absent, the entire retrieved record MUST be ignored. It MUST be the first tag in the list.¶

A DMARC policy record MUST comply with the formal specification found in Section 6.4 in that the "v" tag MUST be present and MUST appear first. Unknown tags MUST be ignored. Syntax errors in the remainder of the record SHOULD be discarded in favor of default values (if any) or ignored outright.¶

Note that given the rules of the previous paragraph, addition of a new tag into the registered list of tags does not itself require a new version of DMARC to be generated (with a corresponding change to the "v" tag's value), but a change to any existing tags does require a new version of DMARC.¶

6.4. Formal Definition

The formal definition of the DMARC format, using [RFC5234], is as follows:¶

[FIXTHIS: Reference to [RFC3986] in code block]¶

  dmarc-uri       = URI
                    ; "URI" is imported from [RFC3986]; commas (ASCII
                    ; 0x2C) and exclamation points (ASCII 0x21)
                    ; MUST be encoded

  dmarc-record    = dmarc-version dmarc-sep *(dmarc-tag dmarc-sep)

  dmarc-tag       = dmarc-request /
                    dmarc-srequest /
                    dmarc-adkim /
                    dmarc-aspf /
                    dmarc-auri /
                    dmarc-furi /
                    dmarc-fo /
                    dmarc-rfmt
                    ; components other than dmarc-version and
                    ; dmarc-request may appear in any order

  dmarc-version   = "v" *WSP "=" *WSP %x44 %x4d %x41 %x52 %x43 %x31

  dmarc-sep       = *WSP %x3b *WSP

  dmarc-request   = "p" *WSP "=" *WSP
                    ( "none" / "quarantine" / "reject" )

  dmarc-srequest  = "sp" *WSP "=" *WSP
                    ( "none" / "quarantine" / "reject" )

  dmarc-adkim     = "adkim" *WSP "=" *WSP ( "r" / "s" )

  dmarc-aspf      = "aspf" *WSP "=" *WSP ( "r" / "s" )

  dmarc-auri      = "rua" *WSP "=" *WSP
                    dmarc-uri *(*WSP "," *WSP dmarc-uri)

  dmarc-furi      = "ruf" *WSP "=" *WSP
                    dmarc-uri *(*WSP "," *WSP dmarc-uri)

  dmarc-fo        = "fo" *WSP "=" *WSP
                    ( "0" / "1" / ( "d" / "s" / "d:s" / "s:d" ) )

  dmarc-rfmt      = "rf"  *WSP "=" *WSP Keyword *(*WSP ":" Keyword)
                    ; registered reporting formats only

"Keyword" is imported from Section 4.1.2 of [RFC5321].¶

6.5. Domain Owner Actions

This section describes Domain Owner actions to fully implement the DMARC mechanism.¶

6.6. PSO Actions

In addition to the DMARC Domain Owner actions, PSOs that require use of DMARC and participate in PSD DMARC ought to make that information availablle to Mail Receivers. [DMARC-PSD] is an experimental method for doing so, and the experiment is described in Appendix A of that document.¶

6.7. Mail Receiver Actions

This section describes receiver actions in the DMARC environment.¶

      if (random mod 100) < pct then
         selected = true
      else
         selected = false

              n!         x          n-x
  f(x) = ----------- *  p  * (1 - p)
         (n-x)! * x!

        5!           1            5-1
   ----------- *  0.2  * (1 - 0.2)     =
   (5-1)! * 1!


      120       1     4
     ----- * 0.2 * 0.8  = 5 * 0.2 * 0.4096 = 0.4096
      24

     0.4096 * 100 = 40.96%

     -----------------------------------
     | X  | Percent chance that X of 5 |
     |    |  will have policy applied  |
     -----------------------------------
     | 0  |        32.768%             |
     -----------------------------------
     | 1  |        40.96%              |
     -----------------------------------
     | 2  |        20.48%              |
     -----------------------------------
     | 3  |         5.12%              |
     -----------------------------------
     | 4  |         0.64%              |
     -----------------------------------
     | 5  |         < 0.1%             |
     -----------------------------------

6.8. Policy Enforcement Considerations

Mail Receivers MAY choose to reject or quarantine email even if email passes the DMARC mechanism check. The DMARC mechanism does not inform Mail Receivers whether an email stream is "good". Mail Receivers are encouraged to maintain anti-abuse technologies to combat the possibility of DMARC-enabled criminal campaigns.¶

Mail Receivers MAY choose to accept email that fails the DMARC mechanism check even if the published Domain Owner Assessment Policy is "reject". Mail Receivers need to make a best effort not to increase the likelihood of accepting abusive mail if they choose not to honor the published Domain Owner Assessment Policy. At a minimum, addition of the Authentication-Results header field (see [RFC8601]) is RECOMMENDED when delivery of failing mail is done. When this is done, the DNS domain name thus recorded MUST be encoded as an A-label.¶

Mail Receivers are only obligated to report reject or quarantine policy actions in aggregate feedback reports that are due to published DMARC Domain Owner Assessment Policy. They are not required to report reject or quarantine actions that are the result of local policy. If local policy information is exposed, abusers can gain insight into the effectiveness and delivery rates of spam campaigns.¶

Final handling of a message is always a matter of local policy. An operator that wishes to favor DMARC policy over SPF policy, for example, will disregard the SPF policy, since enacting an SPF-determined rejection prevents evaluation of DKIM; DKIM might otherwise pass, satisfying the DMARC evaluation. There is a trade-off to doing so, namely acceptance and processing of the entire message body in exchange for the enhanced protection DMARC provides.¶

DMARC-compliant Mail Receivers typically disregard any mail-handling directive discovered as part of an authentication mechanism (e.g., Author Domain Signing Practices (ADSP), SPF) where a DMARC record is also discovered that specifies a policy other than "none". Deviating from this practice introduces inconsistency among DMARC operators in terms of handling of the message. However, such deviation is not proscribed.¶

To enable Domain Owners to receive DMARC feedback without impacting existing mail processing, discovered policies of "p=none" SHOULD NOT modify existing mail handling processes.¶

Mail Receivers MUST also implement reporting instructions of DMARC, even in the absence of a request for DKIM reporting [RFC6651] or SPF reporting [RFC6652]. Furthermore, the presence of such requests SHOULD NOT affect DMARC reporting.¶

9.1. Issues Specific to SPF

Though DMARC does not inherently change the semantics of an SPF policy record, historically lax enforcement of such policies has led many to publish extremely broad records containing many large network ranges. Domain Owners are strongly encouraged to carefully review their SPF records to understand which networks are authorized to send on behalf of the Domain Owner before publishing a DMARC record.¶

Some receiver architectures might implement SPF in advance of any DMARC operations. This means that a "-" prefix on a sender's SPF mechanism, such as "-all", could cause that rejection to go into effect early in handling, causing message rejection before any DMARC processing takes place. Operators choosing to use "-all" should be aware of this.¶

9.2. DNS Load and Caching

DMARC policies are communicated using the DNS and therefore inherit a number of considerations related to DNS caching. The inherent conflict between freshness and the impact of caching on the reduction of DNS-lookup overhead should be considered from the Mail Receiver's point of view. Should Domain Owners or PSOs publish a DNS record with a very short TTL, Mail Receivers can be provoked through the injection of large volumes of messages to overwhelm the publisher's DNS. Although this is not a concern specific to DMARC, the implications of a very short TTL should be considered when publishing DMARC policies.¶

Conversely, long TTLs will cause records to be cached for long periods of time. This can cause a critical change to DMARC parameters advertised by a Domain Owner or PSO to go unnoticed for the length of the TTL (while waiting for DNS caches to expire). Avoiding this problem can mean shorter TTLs, with the potential problems described above. A balance should be sought to maintain responsiveness of DMARC preference changes while preserving the benefits of DNS caching.¶

9.3. Rejecting Messages

This proposal calls for rejection of a message during the SMTP session under certain circumstances. This is preferable to generation of a Delivery Status Notification ([RFC3464]), since fraudulent messages caught and rejected using DMARC would then result in annoying generation of such failure reports that go back to the RFC5321.MailFrom address.¶

This synchronous rejection is typically done in one of two ways:¶

• Full rejection, wherein the SMTP server issues a 5xy reply code as an indication to the SMTP client that the transaction failed; the SMTP client is then responsible for generating notification that delivery failed (see Section 4.2.5 of [RFC5321]).¶

• A "silent discard", wherein the SMTP server returns a 2xy reply code implying to the client that delivery (or, at least, relay) was successfully completed, but then simply discarding the message with no further action.¶

Each of these has a cost. For instance, a silent discard can help to prevent backscatter, but it also effectively means that the SMTP server has to be programmed to give a false result, which can confound external debugging efforts.¶

Similarly, the text portion of the SMTP reply may be important to consider. For example, when rejecting a message, revealing the reason for the rejection might give an attacker enough information to bypass those efforts on a later attempt, though it might also assist a legitimate client to determine the source of some local issue that caused the rejection.¶

In the latter case, when doing an SMTP rejection, providing a clear hint can be useful in resolving issues. A receiver might indicate in plain text the reason for the rejection by using the word "DMARC" somewhere in the reply text. Many systems are able to scan the SMTP reply text to determine the nature of the rejection. Thus, providing a machine-detectable reason for rejection allows the problems causing rejections to be properly addressed by automated systems. For example:¶

550 5.7.1 Email rejected per DMARC policy for example.com

If a Mail Receiver elects to defer delivery due to inability to retrieve or apply DMARC policy, this is best done with a 4xy SMTP reply code.¶

9.4. Identifier Alignment Considerations

The DMARC mechanism allows both DKIM and SPF-authenticated identifiers to authenticate email on behalf of a Domain Owner and, possibly, on behalf of different subdomains. If malicious or unaware users can gain control of the SPF record or DKIM selector records for a subdomain, the subdomain can be used to generate DMARC-passing email on behalf of the Organizational Domain.¶

For example, an attacker who controls the SPF record for "evil.example.com" can send mail with an RFC5322.From header field containing "foo@example.com" that can pass both authentication and the DMARC check against "example.com".¶

The Organizational Domain administrator should be careful not to delegate control of subdomains if this is an issue, and to consider using the "strict" Identifier Alignment option if appropriate.¶

9.5. Interoperability Issues

DMARC limits which end-to-end scenarios can achieve a "pass" result.¶

Because DMARC relies on [RFC7208] and/or [RFC6376] to achieve a "pass", their limitations also apply.¶

Additional DMARC constraints occur when a message is processed by some Mediators, such as mailing lists. Transiting a Mediator often causes either the authentication to fail or Identifier Alignment to be lost. These transformations may conform to standards but will still prevent a DMARC "pass".¶

In addition to Mediators, mail that is sent by authorized, independent third parties might not be sent with Identifier Alignment, also preventing a "pass" result.¶

Issues specific to the use of policy mechanisms alongside DKIM are further discussed in [RFC6377], particularly Section 5.2.¶

10.1. Authentication-Results Method Registry Update

IANA has added the following to the "Email Authentication Methods" registry:¶

Method: dmarc¶

Defined: RFC 7489¶

ptype: header¶

Property: from¶

Value: the domain portion of the RFC5322.From header field¶

Status: active¶

Version: 1¶

Method: dmarc¶

Defined: RFC 7489¶

ptype: polrec¶

Property: p¶

Value: the p= value read from the discovered policy record¶

Status: active¶

Version: 1¶

Method: dmarc¶

Defined: RFC 7489¶

ptype: polrec¶

Property: domain¶

Value: the domain at which the policy record was discovered, if different from the RFC5322.From domain¶

Status: active¶

Version: 1¶

10.2. Authentication-Results Result Registry Update

IANA has added the following in the "Email Authentication Result Names" registry:¶

Code: none¶

Existing/New Code: existing¶

Defined: [RFC8601]¶

Auth Method: dmarc (added)¶

Meaning:

No DMARC policy record was published for the aligned identifier, or no aligned identifier could be extracted.¶

Status: active¶

Code: pass¶

Existing/New Code: existing¶

Defined: [RFC8601]¶

Auth Method: dmarc (added)¶

Meaning:

A DMARC policy record was published for the aligned identifier, and at least one of the authentication mechanisms passed.¶

Status: active¶

Code: fail¶

Existing/New Code: existing¶

Defined: [RFC8601]¶

Auth Method: dmarc (added)¶

Meaning:

A DMARC policy record was published for the aligned identifier, and none of the authentication mechanisms passed.¶

Status: active¶

Code: temperror¶

Existing/New Code: existing¶

Defined: [RFC8601]¶

Auth Method: dmarc (added)¶

Meaning:

A temporary error occurred during DMARC evaluation. A later attempt might produce a final result.¶

Status: active¶

Code: permerror¶

Existing/New Code: existing¶

Defined: [RFC8601]¶

Auth Method: dmarc (added)¶

Meaning:

A permanent error occurred during DMARC evaluation, such as encountering a syntactically incorrect DMARC record. A later attempt is unlikely to produce a final result.¶

Status: active¶

10.3. Feedback Report Header Fields Registry Update

The following has been added to the "Feedback Report Header Fields" registry:¶

Field Name: Identity-Alignment¶

Description:

indicates whether the message about which a report is being generated had any identifiers in alignment as defined in RFC 7489¶

Multiple Appearances: No¶

Related "Feedback-Type": auth-failure¶

Reference: RFC 7489¶

Status: current¶

10.4. DMARC Tag Registry

A new registry tree called "Domain-based Message Authentication, Reporting, and Conformance (DMARC) Parameters" has been created. Within it, a new sub-registry called the "DMARC Tag Registry" has been created.¶

Names of DMARC tags must be registered with IANA in this new sub-registry. New entries are assigned only for values that have been documented in a manner that satisfies the terms of Specification Required, per [RFC8126]. Each registration must include the tag name; the specification that defines it; a brief description; and its status, which must be one of "current", "experimental", or "historic". The Designated Expert needs to confirm that the provided specification adequately describes the new tag and clearly presents how it would be used within the DMARC context by Domain Owners and Mail Receivers.¶

To avoid version compatibility issues, tags added to the DMARC specification are to avoid changing the semantics of existing records when processed by implementations conforming to prior specifications.¶

The initial set of entries in this registry is as follows:¶

10.5. DMARC Report Format Registry

Also within "Domain-based Message Authentication, Reporting, and Conformance (DMARC) Parameters", a new sub-registry called "DMARC Report Format Registry" has been created.¶

Names of DMARC failure reporting formats must be registered with IANA in this registry. New entries are assigned only for values that satisfy the definition of Specification Required, per [RFC8126]. In addition to a reference to a permanent specification, each registration must include the format name; a brief description; and its status, which must be one of "current", "experimental", or "historic". The Designated Expert needs to confirm that the provided specification adequately describes the report format and clearly presents how it would be used within the DMARC context by Domain Owners and Mail Receivers.¶

The initial entry in this registry is as follows:¶

10.6. Underscored and Globally Scoped DNS Node Names Registry

Per [!@RFC8552], please add the following entry to the "Underscored and Globally Scoped DNS Node Names" registry:¶

11.1. Authentication Methods

Security considerations from the authentication methods used by DMARC are incorporated here by reference.¶

11.2. Attacks on Reporting URIs

URIs published in DNS TXT records are well-understood possible targets for attack. Specifications such as [RFC1035] and [RFC2142] either expose or cause the exposure of email addresses that could be flooded by an attacker, for example; MX, NS, and other records found in the DNS advertise potential attack destinations; common DNS names such as "www" plainly identify the locations at which particular services can be found, providing destinations for targeted denial-of-service or penetration attacks.¶

Thus, Domain Owners will need to harden these addresses against various attacks, including but not limited to:¶

• high-volume denial-of-service attacks;¶

• deliberate construction of malformed reports intended to identify or exploit parsing or processing vulnerabilities;¶

• deliberate construction of reports containing false claims for the Submitter or Reported-Domain fields, including the possibility of false data from compromised but known Mail Receivers.¶

11.3. DNS Security

The DMARC mechanism and its underlying technologies (SPF, DKIM) depend on the security of the DNS. If hostile parties can snoop on DNS traffic, they can get an idea of who is sending mail. If they can block outgoing or reply DNS messages, they can prevent systems from discovering senders' DMARC policies, causing recipients to assume p=none by default/ If they can send forged response packets, they can make aligned mail appear unaligned or vice-versa.¶

None of these threats are unique to DMARC, and they can be addressed using a variety of techniques. Signing DNS records with DNSSEC [RFC4033] enables recipients to detect and discard forged responses. DNS over TLS [RFC7858] or DNS over HTTPS [RFC8484] can mitigate snooping and forged responses.¶

11.4. Display Name Attacks

A common attack in messaging abuse is the presentation of false information in the display-name portion of the RFC5322.From header field. For example, it is possible for the email address in that field to be an arbitrary address or domain name, while containing a well-known name (a person, brand, role, etc.) in the display name, intending to fool the end user into believing that the name is used legitimately. The attack is predicated on the notion that most common MUAs will show the display name and not the email address when both are available.¶

Generally, display name attacks are out of scope for DMARC, as further exploration of possible defenses against these attacks needs to be undertaken.¶

There are a few possible mechanisms that attempt mitigation of these attacks, such as the following:¶

• If the display name is found to include an email address (as specified in [RFC5322]), execute the DMARC mechanism on the domain name found there rather than the domain name discovered originally. However, this addresses only a very specific attack space, and spoofers can easily circumvent it by simply not using an email address in the display name. There are also known cases of legitimate uses of an email address in the display name with a domain different from the one in the address portion, e.g.,¶

  From: "user@example.org via Bug Tracker" support@example.com¶

• In the MUA, only show the display name if the DMARC mechanism succeeds. This too is easily defeated, as an attacker could arrange to pass the DMARC tests while fraudulently using another domain name in the display name.¶

• In the MUA, only show the display name if the DMARC mechanism passes and the email address thus validated matches one found in the receiving user's list of known addresses.¶

11.5. External Reporting Addresses

To avoid abuse by bad actors, reporting addresses generally have to be inside the domains about which reports are requested. In order to accommodate special cases such as a need to get reports about domains that cannot actually receive mail, The DMARC reporting documents describe a DNS-based mechanism for verifying approved external reporting.¶

The obvious consideration here is an increased DNS load against domains that are claimed as external recipients. Negative caching will mitigate this problem, but only to a limited extent, mostly dependent on the default TTL in the domain's SOA record.¶

Where possible, external reporting is best achieved by having the report be directed to domains that can receive mail and simply having it automatically forwarded to the desired external destination.¶

Note that the addresses shown in the "ruf" tag receive more information that might be considered private data, since it is possible for actual email content to appear in the failure reports. The URIs identified there are thus more attractive targets for intrusion attempts than those found in the "rua" tag. Moreover, attacking the DNS of the subject domain to cause failure data to be routed fraudulently to an attacker's systems may be an attractive prospect. Deployment of [RFC4033] is advisable if this is a concern.¶

The verification mechanism presented in the DMARC reporting docuemnts is currently not mandatory ("MUST") but strongly recommended ("SHOULD"). It is possible that it would be elevated to a "MUST" by later security review.¶

11.6. Secure Protocols

This document encourages use of secure transport mechanisms to prevent loss of private data to third parties that may be able to monitor such transmissions. Unencrypted mechanisms should be avoided.¶

In particular, a message that was originally encrypted or otherwise secured might appear in a report that is not sent securely, which could reveal private information.¶

A.1. S/MIME

S/MIME, or Secure Multipurpose Internet Mail Extensions, is a standard for encryption and signing of MIME data in a message. This was suggested and considered as a third security protocol for authenticating the source of a message.¶

DMARC is focused on authentication at the domain level (i.e., the Domain Owner taking responsibility for the message), while S/MIME is really intended for user-to-user authentication and encryption. This alone appears to make it a bad fit for DMARC's goals.¶

S/MIME also suffers from the heavyweight problem of Public Key Infrastructure, which means that distribution of keys used to verify signatures needs to be incorporated. In many instances, this alone is a showstopper. There have been consistent promises that PKI usability and deployment will improve, but these have yet to materialize. DMARC can revisit this choice after those barriers are addressed.¶

S/MIME has extensive deployment in specific market segments (government, for example) but does not enjoy similar widespread deployment over the general Internet, and this shows no signs of changing. DKIM and SPF both are deployed widely over the general Internet, and their adoption rates continue to be positive.¶

Finally, experiments have shown that including S/MIME support in the initial version of DMARC would neither cause nor enable a substantial increase in the accuracy of the overall mechanism.¶

A.2. Method Exclusion

It was suggested that DMARC include a mechanism by which a Domain Owner could tell Message Receivers not to attempt validation by one of the supported methods (e.g., "check DKIM, but not SPF").¶

Specifically, consider a Domain Owner that has deployed one of the technologies, and that technology fails for some messages, but such failures don't cause enforcement action. Deploying DMARC would cause enforcement action for policies other than "none", which would appear to exclude participation by that Domain Owner.¶

The DMARC development team evaluated the idea of policy exception mechanisms on several occasions and invariably concluded that there was not a strong enough use case to include them. The specific target audience for DMARC does not appear to have concerns about the failure modes of one or the other being a barrier to DMARC's adoption.¶

In the scenario described above, the Domain Owner has a few options:¶

1. Tighten up its infrastructure to minimize the failure modes of the single deployed technology.¶

2. Deploy the other supported authentication mechanism, to offset the failure modes of the first.¶

3. Deploy DMARC in a reporting-only mode.¶

A.3. Sender Header Field

It has been suggested in several message authentication efforts that the Sender header field be checked for an identifier of interest, as the standards indicate this as the proper way to indicate a re-mailing of content such as through a mailing list. Most recently, it was a protocol-level option for DomainKeys, but on evolution to DKIM, this property was removed.¶

The DMARC development team considered this and decided not to include support for doing so, for the following reasons:¶

1. The main user protection approach is to be concerned with what the user sees when a message is rendered. There is no consistent behavior among MUAs regarding what to do with the content of the Sender field, if present. Accordingly, supporting checking of the Sender identifier would mean applying policy to an identifier the end user might never actually see, which can create a vector for attack against end users by simply forging a Sender field containing some identifier that DMARC will like.¶

2. Although it is certainly true that this is what the Sender field is for, its use in this way is also unreliable, making it a poor candidate for inclusion in the DMARC evaluation algorithm.¶

3. Allowing multiple ways to discover policy introduces unacceptable ambiguity into the DMARC evaluation algorithm in terms of which policy is to be applied and when.¶

A.4. Domain Existence Test

A common practice among MTA operators, and indeed one documented in [RFC5617], is a test to determine domain existence prior to any more expensive processing. This is typically done by querying the DNS for MX, A, or AAAA resource records for the name being evaluated and assuming that the domain is nonexistent if it could be determined that no such records were published for that domain name.¶

The original pre-standardization version of this protocol included a mandatory check of this nature. It was ultimately removed, as the method's error rate was too high without substantial manual tuning and heuristic work. There are indeed use cases this work needs to address where such a method would return a negative result about a domain for which reporting is desired, such as a registered domain name that never sends legitimate mail and thus has none of these records present in the DNS.¶

A.5. Issues with ADSP in Operation

DMARC has been characterized as a "super-ADSP" of sorts.¶

Contributors to DMARC have compiled a list of issues associated with ADSP, gained from operational experience, that have influenced the direction of DMARC:¶

1. ADSP has no support for subdomains, i.e., the ADSP record for example.com does not explicitly or implicitly apply to subdomain.example.com. If wildcarding is not applied, then spammers can trivially bypass ADSP by sending from a subdomain with no ADSP record.¶

2. Nonexistent subdomains are explicitly out of scope in ADSP. There is nothing in ADSP that states receivers should simply reject mail from NXDOMAINs regardless of ADSP policy (which of course allows spammers to trivially bypass ADSP by sending email from nonexistent subdomains).¶

3. ADSP has no operational advice on when to look up the ADSP record.¶

4. ADSP has no support for using SPF as an auxiliary mechanism to DKIM.¶

5. ADSP has no support for a slow rollout, i.e., no way to configure a percentage of email on which the receiver should apply the policy. This is important for large-volume senders.¶

6. ADSP has no explicit support for an intermediate phase where the receiver quarantines (e.g., sends to the recipient's "spam" folder) rather than rejects the email.¶

7. The binding between the "From" header domain and DKIM is too tight for ADSP; they must match exactly.¶

A.6. Organizational Domain Discovery Issues

Although protocols like ADSP are useful for "protecting" a specific domain name, they are not helpful at protecting subdomains. If one wished to protect "example.com" by requiring via ADSP that all mail bearing an RFC5322.From domain of "example.com" be signed, this would "protect" that domain; however, one could then craft an email whose RFC5322.From domain is "security.example.com", and ADSP would not provide any protection. One could use a DNS wildcard, but this can undesirably interfere with other DNS activity; one could add ADSP records as fraudulent domains are discovered, but this solution does not scale and is a purely reactive measure against abuse.¶

The DNS does not provide a method by which the "domain of record", or the domain that was actually registered with a domain registrar, can be determined given an arbitrary domain name. Suggestions have been made that attempt to glean such information from SOA or NS resource records, but these too are not fully reliable, as the partitioning of the DNS is not always done at administrative boundaries.¶

When seeking domain-specific policy based on an arbitrary domain name, one could "climb the tree", dropping labels off the left end of the name until the root is reached or a policy is discovered, but then one could craft a name that has a large number of nonsense labels; this would cause a Mail Receiver to attempt a large number of queries in search of a policy record. Sending many such messages constitutes an amplified denial-of-service attack.¶

The Organizational Domain mechanism is a necessary component to the goals of DMARC. The method described in Section 3.15 is far from perfect but serves this purpose reasonably well without adding undue burden or semantics to the DNS. If a method is created to do so that is more reliable and secure than the use of a public suffix list, DMARC should be amended to use that method as soon as it is generally available.¶

B.1. Identifier Alignment Examples

The following examples illustrate the DMARC mechanism's use of Identifier Alignment. For brevity's sake, only message headers are shown, as message bodies are not considered when conducting DMARC checks.¶

     MAIL FROM: <sender@example.com>

     From: sender@example.com
     Date: Fri, Feb 15 2002 16:54:30 -0800
     To: receiver@example.org
     Subject: here's a sample

     MAIL FROM: <sender@child.example.com>

     From: sender@example.com
     Date: Fri, Feb 15 2002 16:54:30 -0800
     To: receiver@example.org
     Subject: here's a sample

     MAIL FROM: <sender@example.net>

     From: sender@child.example.com
     Date: Fri, Feb 15 2002 16:54:30 -0800
     To: receiver@example.org
     Subject: here's a sample

     DKIM-Signature: v=1; ...; d=example.com; ...
     From: sender@example.com
     Date: Fri, Feb 15 2002 16:54:30 -0800
     To: receiver@example.org
     Subject: here's a sample

     DKIM-Signature: v=1; ...; d=example.com; ...
     From: sender@child.example.com
     Date: Fri, Feb 15 2002 16:54:30 -0800
     To: receiver@example.org
     Subject: here's a sample

     DKIM-Signature: v=1; ...; d=sample.net; ...
     From: sender@child.example.com
     Date: Fri, Feb 15 2002 16:54:30 -0800
     To: receiver@example.org
     Subject: here's a sample

B.2. Domain Owner Example

A Domain Owner that wants to use DMARC should have already deployed and tested SPF and DKIM. The next step is to publish a DNS record that advertises a DMARC policy for the Domain Owner's Organizational Domain.¶

  % dig +short TXT _dmarc.example.com.
  "v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com"

  ; DMARC record for the domain example.com

  _dmarc  IN TXT ( "v=DMARC1; p=none; "
                   "rua=mailto:dmarc-feedback@example.com" )

  % dig +short TXT _dmarc.example.com.
  "v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
   ruf=mailto:auth-reports@example.com"

  ; DMARC record for the domain example.com

  _dmarc  IN TXT ( "v=DMARC1; p=none; "
                    "rua=mailto:dmarc-feedback@example.com; "
                    "ruf=mailto:auth-reports@example.com" )

  % dig +short TXT _dmarc.example.com.
  "v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
   ruf=mailto:auth-reports@thirdparty.example.net"

  ; DMARC record for the domain example.com

  _dmarc IN TXT ( "v=DMARC1; p=none; "
                  "rua=mailto:dmarc-feedback@example.com; "
                  "ruf=mailto:auth-reports@thirdparty.example.net" )

  % dig +short TXT example.com._report._dmarc.thirdparty.example.net
  "v=DMARC1;"

  ; zone file for thirdparty.example.net
  ; Accept DMARC failure reports on behalf of example.com

  example.com._report._dmarc   IN   TXT    "v=DMARC1;"

  % dig +short TXT _dmarc.test.example.com
  "v=DMARC1; p=quarantine; rua=mailto:dmarc-feedback@example.com,
   mailto:tld-test@thirdparty.example.net pct=25"

  ; DMARC record for the domain test.example.com

  _dmarc IN  TXT  ( "v=DMARC1; p=quarantine; "
                    "rua=mailto:dmarc-feedback@example.com,"
                    "mailto:tld-test@thirdparty.example.net;"
                    "pct=25" )

  % dig +short TXT _dmarc.test.example.com
  "v=DMARC1; p=reject; rua=mailto:dmarc-feedback@example.com,
   mailto:tld-test@thirdparty.example.net"

  ; DMARC record for the domain test.example.com

  _dmarc IN  TXT  ( "v=DMARC1; p=reject; "
                    "rua=mailto:dmarc-feedback@example.com,"
                    "mailto:tld-test@thirdparty.example.net" )

B.3. Mail Receiver Example

A Mail Receiver that wants to use DMARC should already be checking SPF and DKIM, and possess the ability to collect relevant information from various email-processing stages to provide feedback to Domain Owners (possibly via Report Receivers).¶

B.4. Processing of SMTP Time

An optimal DMARC-enabled Mail Receiver performs authentication and Identifier Alignment checking during the [RFC5321] conversation.¶

Prior to returning a final reply to the DATA command, the Mail Receiver's MTA has performed:¶

1. An SPF check to determine an SPF-authenticated Identifier.¶

2. DKIM checks that yield one or more DKIM-authenticated Identifiers.¶

3. A DMARC policy lookup.¶

The presence of an Author Domain DMARC record indicates that the Mail Receiver should continue with DMARC-specific processing before returning a reply to the DATA command.¶

Given a DMARC record and the set of Authenticated Identifiers, the Mail Receiver checks to see if the Authenticated Identifiers align with the Author Domain (taking into consideration any strict versus relaxed options found in the DMARC record).¶

For example, the following sample data is considered to be from a piece of email originating from the Domain Owner of "example.com":¶

  Author Domain: example.com
  SPF-authenticated Identifier: mail.example.com
  DKIM-authenticated Identifier: example.com
  DMARC record:
    "v=DMARC1; p=reject; aspf=r;
     rua=mailto:dmarc-feedback@example.com"

In the above sample, both the SPF-authenticated Identifier and the DKIM-authenticated Identifier align with the Author Domain. The Mail Receiver considers the above email to pass the DMARC check, avoiding the "reject" policy that is requested to be applied to email that fails to pass the DMARC check.¶

If no Authenticated Identifiers align with the Author Domain, then the Mail Receiver applies the DMARC-record-specified policy. However, before this action is taken, the Mail Receiver can consult external information to override the Domain Owner's Assessment Policy.
For example, if the Mail Receiver knows that this particular email came from a known and trusted forwarder (that happens to break both SPF and DKIM), then the Mail Receiver may choose to ignore the Domain Owner's policy.¶

The Mail Receiver is now ready to reply to the DATA command. If the DMARC check yields that the message is to be rejected, then the Mail Receiver replies with a 5xy code to inform the sender of failure. If the DMARC check cannot be resolved due to transient network errors, then the Mail Receiver replies with a 4xy code to inform the sender as to the need to reattempt delivery later. If the DMARC check yields a passing message, then the Mail Receiver continues on with email processing, perhaps using the result of the DMARC check as an input to additional processing modules such as a domain reputation query.¶

Before exiting DMARC-specific processing, the Mail Receiver checks to see if the Author Domain DMARC record requests AFRF-based reporting. If so, then the Mail Receiver can emit an AFRF to the reporting address supplied in the DMARC record.¶

At the exit of DMARC-specific processing, the Mail Receiver captures (through logging or direct insertion into a data store) the result of DMARC processing. Captured information is used to build feedback for Domain Owner consumption. This is not necessary if the Domain Owner has not requested aggregate reports, i.e., no "rua" tag was found in the policy record.¶

B.5. Utilization of Aggregate Feedback: Example

Aggregate feedback is consumed by Domain Owners to verify a Domain Owner's understanding of how the Domain Owner's domain is being processed by the Mail Receiver. Aggregate reporting data on emails that pass all DMARC-supporting authentication checks is used by Domain Owners to verify that authentication practices remain accurate. For example, if a third party is sending on behalf of a Domain Owner, the Domain Owner can use aggregate report data to verify ongoing authentication practices of the third party.¶

Data on email that only partially passes underlying authentication checks provides visibility into problems that need to be addressed by the Domain Owner. For example, if either SPF or DKIM fails to pass, the Domain Owner is provided with enough information to either directly correct the problem or understand where authentication- breaking changes are being introduced in the email transmission path. If authentication-breaking changes due to email transmission path cannot be directly corrected, then the Domain Owner at least maintains an understanding of the effect of DMARC-based policies upon the Domain Owner's email.¶

Data on email that fails all underlying authentication checks provides baseline visibility on how the Domain Owner's domain is being received at the Mail Receiver. Based on this visibility, the Domain Owner can begin deployment of authentication technologies across uncovered email sources. Additionally, the Domain Owner may come to an understanding of how its domain is being misused.¶

(Aggregate report example should be moved to [DMARC-Aggregate-Reporting])¶

C.1. January 5, 2021

C.2. February 4, 2021

C.3. February 10, 2021

C.4. March 1, 2021

C.5. March 8, 2021

C.6. March 16, 2021

C.7. March 23, 2021

C.8. March 29, 2021

C.9. April 12, 2021

C.10. April 13, 2021

C.11. April 14, 2021

C.12. April 15, 2021

C.13. April 19, 2021

C.14. April 20, 2021

C.15. April 22, 2021

C.16. June 16, 2021