



Network Working Group                                         R. Clayton
Internet-Draft                                                     Yahoo
Intended status: Standards Track                               W. Chuang
Expires: 28 February 2026                                         Google
                                                             B. Gondwana
                                                        Fastmail Pty Ltd
                                                          27 August 2025


            DomainKeys Identified Mail Signatures v2 (DKIM2)
                      draft-clayton-dkim2-spec-00

Abstract

   DomainKeys Identified Mail v2 (DKIM2) permits a person, role, or
   organization that owns a signing domain to document that it has
   handled an email message by associating their domain with the
   message.  This is achieved by applying a cryptographic signature to
   the message.  Verification is performed by querying an entry within
   the signing domain's DNS space to retrieve an appropriate public key.
   As a message is transferred from author to recipient further
   signatures will be added to provide a validatable "chain".  This
   permits validators to identify when messages have been unexpectedly
   "replayed" and can ensure that delivery status notifications are only
   sent to entities that were involved in the transmission of a message.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on 28 February 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  DKIM2 Architecture Documents  . . . . . . . . . . . . . .   4
   2.  Terminology and Definitions . . . . . . . . . . . . . . . . .   4
     2.1.  Forwarder . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Signers . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Verifiers . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Signing Domain  . . . . . . . . . . . . . . . . . . . . .   5
     2.5.  Whitespace  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.6.  Imported ABNF Tokens  . . . . . . . . . . . . . . . . . .   5
     2.7.  Common ABNF Tokens  . . . . . . . . . . . . . . . . . . .   6
   3.  Protocol Elements . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Selectors . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Tag=Value Lists . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Signing and Verification Algorithms . . . . . . . . . . .   8
       3.3.1.  The RSA-SHA256 Signing Algorithm  . . . . . . . . . .   8
       3.3.2.  The Ed25519-SHA256 Signing Algorithm  . . . . . . . .   8
       3.3.3.  Other Algorithms  . . . . . . . . . . . . . . . . . .   9
     3.4.  Canonicalization  . . . . . . . . . . . . . . . . . . . .   9
       3.4.1.  The Header Canonicalization Algorithm . . . . . . . .   9
       3.4.2.  The Body Canonicalization Algorithm . . . . . . . . .  10
     3.5.  The DKIM2-Signature Header Field  . . . . . . . . . . . .  11
     3.6.  Key Management  . . . . . . . . . . . . . . . . . . . . .  16
     3.7.  Computing the Message Hashes  . . . . . . . . . . . . . .  16
     3.8.  Input Requirements  . . . . . . . . . . . . . . . . . . .  17
     3.9.  Output Requirements . . . . . . . . . . . . . . . . . . .  18
   4.  Semantics of Multiple Signatures  . . . . . . . . . . . . . .  18
   5.  Signer Actions  . . . . . . . . . . . . . . . . . . . . . . .  19
     5.1.  Document modifications made to a message  . . . . . . . .  19
     5.2.  Determine what RFC5321 "envelope" will be used for the
           message . . . . . . . . . . . . . . . . . . . . . . . . .  19
     5.3.  Select a Private Key and Corresponding Selector
           Information . . . . . . . . . . . . . . . . . . . . . . .  19
     5.4.  Normalize the Message to Prevent Transport Conversions  .  20
     5.5.  Compute the Message Hash and
           Signature{#signer-compute}  . . . . . . . . . . . . . . .  20
     5.6.  Insert the DKIM2-Signature Header Field . . . . . . . . .  20
   6.  Verifier Actions  . . . . . . . . . . . . . . . . . . . . . .  21



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     6.1.  Extract Signatures from the Message . . . . . . . . . . .  21
       6.1.1.  Validate the Signature Header Field . . . . . . . . .  22
       6.1.2.  Get the Public Key  . . . . . . . . . . . . . . . . .  22
       6.1.3.  Compute the Verification  . . . . . . . . . . . . . .  23
     6.2.  Communicate Verification Results  . . . . . . . . . . . .  24
     6.3.  Interpret Results/Apply Local Policy  . . . . . . . . . .  24
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  27
   Appendix A.  Changes from Earlier Versions  . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   DomainKeys Identified Mail v2 (DKIM2) permits a person, role, or
   organization to document that they have handled an email message by
   associating a domain name [RFC1034] with the message [RFC5322].  A
   public key signature is used to record that they have been able to
   read the contents of the message and write to it.

   Verification of claims is achieved by fetching a public key stored in
   the DNS under the relevant domain and then checking the signature.

   Message transit from author to recipient is through Forwarders that
   typically make no substantive change to the message content and thus
   preserve the DKIM2 signature.  Where they do make a change the
   changes they have made are documented so that these can be "undone"
   and the original signature validated.

   When a message is forwarded from one system to another an additional
   DKIM2 signature is added on each occasion.  This chain of custody
   assists validators in distinguishing between messages that were
   intended to be sent to a particular email address and those that are
   being "replayed" to that address.

   The chain of custody can also be used to ensure that delivery status
   notifications are only sent to entities that were involved in the
   transmission of a message.

   Organizations that process a message can add to their signature a
   request for feedback as to any opinion (for example, that it was
   considered to be spam) that the eventual recipient of the message
   wishes to share.






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1.1.  DKIM2 Architecture Documents

   Readers are advised to be familiar with the material in TBA, TBA and
   TBA which provide the background for the development of DKIM2, an
   overview of the service, and deployment and operations guidance and
   advice.

2.  Terminology and Definitions

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

   DKIM2 is designed to operate within the Internet Mail service, as
   defined in [RFC5598].  Basic email terminology is taken from that
   specification.

   DKIM2 inherits many ideas from DKIM ([RFC6376]) which, for clarity we
   refer to in this specification as DKIM1.

   Syntax descriptions use Augmented BNF (ABNF) [RFC5234].

   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
   [RFC2119].  These words take their normative meanings only when they
   are presented in ALL UPPERCASE.

2.1.  Forwarder

   [RFC5598] defines a Relay as transmitting or retransmitting a message
   but states that it will not modify the envelope information or the
   message content semantics.  It also defines a Gateway as a hybrid of
   User and Relay that connects heterogeneous mail services, In this
   document we use the concept of a Forwarder which is an MTA that
   receives a message and then either delivers it into a destination
   mailbox or forwards it on to another system in an automated, pre-
   determined, manner.

   As will be seen, Forwarders may alter message content or envelopes
   but will create a signed record of what they have done.

2.2.  Signers

   Elements in the mail system that sign messages on behalf of a domain
   are referred to as Signers.  These may be MUAs (Mail User Agents),
   MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other
   agents such as mailing list "exploders".  In general, any Signer will
   be involved in the injection of a message into the message system in
   some way.  The key point is that a message must be signed before it



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   leaves the administrative domain of the Signer.

2.3.  Verifiers

   Elements in the mail system that verify signatures are referred to as
   Verifiers.  These may be Forwarders, MTAs, Mail Delivery Agents
   (MDAs), or MUAs.  It is an expectation of DKIM2 that a recipient of a
   message will wish to verify some or all signatures before determining
   whether or not to accept the message or pass it on to another entity.

2.4.  Signing Domain

   A domain name associated with a signature.  This domain may be
   associated with the author of an email, their organization, a company
   hired to deliver the email, a mailing list operator, or some other
   entity that handles email.  What they have in common is that at some
   point they had access to the entire contents of the email and were in
   a position to add their signature to the email.

2.5.  Whitespace

   There are two forms of whitespace used in this specification:

   *  WSP represents simple whitespace, i.e., a space or a tab character
      (formal definition in [RFC5234]).

   *  FWS is folding whitespace.  It allows multiple lines separated by
      CRLF followed by at least one whitespace, to be joined.

   The formal ABNF for these are (WSP given for information only):

   WSP =   SP / HTAB
   FWS =   [*WSP CRLF] 1*WSP

   The definition of FWS is identical to that in [RFC5322] except for
   the exclusion of obs-FWS.

2.6.  Imported ABNF Tokens

   The following tokens are imported from other RFCs as noted.  Those
   RFCs should be considered definitive.

   The following tokens are imported from [RFC5321]:

   *  "local-part" (implementation warning: this permits quoted strings)

   *  "sub-domain"




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   The following tokens are imported from [RFC5322]:

   *  "field-name" (name of a header field)

   Other tokens not defined herein are imported from [RFC5234].  These
   are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF,
   etc.

2.7.  Common ABNF Tokens

   The following ABNF tokens are used elsewhere in this document:

   ALPHADIGITPS =  (ALPHA / DIGIT / "+" / "/")
   base64string =  ALPHADIGITPS *([FWS] ALPHADIGITPS)
                   [ [FWS] "=" [ [FWS] "=" ] ]

3.  Protocol Elements

   Protocol Elements are conceptual parts of the protocol that are not
   specific to either Signers or Verifiers.  The protocol descriptions
   for Signers and Verifiers are described in later sections ("Signer
   Actions" (Section 5) and "Verifier Actions" (Section 6).  NOTE: This
   section must be read in the context of those sections.

3.1.  Selectors

   To support multiple concurrent public keys per signing domain, the
   key namespace is subdivided using "selectors".

   Periods are allowed in selectors and are component separators.
   Periods in selectors define DNS label boundaries in a manner similar
   to the conventional use in domain names.  This will allow portions of
   the selector namespace to be delegated.

   ABNF:

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

   The number of public keys and corresponding selectors for each domain
   is determined by the domain owner.  Many domain owners will be
   satisfied with just one selector, whereas administratively
   distributed organizations can choose to manage disparate selectors
   and key pairs in different regions or on different email servers.

   Beyond administrative convenience, selectors make it possible to
   seamlessly replace public keys on a routine basis.  If a domain
   wishes to change from using a public key associated with selector
   "january2026" to a public key associated with selector



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   "february2026", it merely makes sure that both public keys are
   advertised in the public key repository concurrently for the
   transition period during which email may be in transit prior to
   verification.  At the start of the transition period, the outbound
   email servers are configured to sign with the "february2026" private
   key.  At the end of the transition period, the "january2026" public
   key is removed from the public key repository.

   INFORMATIVE NOTE: A key may also be revoked as described below.  The
   distinction between revoking and removing a key selector record is
   subtle.  When phasing out keys as described above, a signing domain
   would probably simply remove the key record after the transition
   period.  However, a signing domain could elect to revoke the key (but
   maintain the key record) for a further period.  There is no defined
   semantic difference between a revoked key and a removed key.

3.2.  Tag=Value Lists

   DKIM2 uses a simple "tag=value" syntax in several contexts, including
   in messages and domain signature records (see [DKIMKEYS]).

   Values are a series of strings containing either plain text or
   "base64" text (as defined in [RFC2045], Section 6.8).  The name of
   the tag will determine the encoding of each value.  Unencoded
   semicolon (";") characters MUST NOT occur in the tag value, since
   that separates tag-specs.

   INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" defined
   below (as "tag-value") only includes 7-bit characters, an
   implementation that wished to anticipate future standards would be
   advised not to preclude the use of UTF-8-encoded ([RFC3629]) text in
   tag=value lists.

   Formally, the ABNF syntax rules are as follows:

   tag-list  =  tag-spec *( ";" tag-spec ) [ ";" ]
   tag-spec  =  [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
   tag-name  =  ALPHA *ALNUMPUNC
   tag-value =  [ tval *( 1*(WSP / FWS) tval ) ]
                     ; Prohibits WSP and FWS at beginning and end
   tval      =  1*VALCHAR
   VALCHAR   =  %x21-3A / %x3C-7E
                     ; EXCLAMATION to TILDE except SEMICOLON
   ALNUMPUNC =  ALPHA / DIGIT / "_"

   Note that WSP is allowed anywhere around tags.  In particular, any
   WSP after the "=" and any WSP before the terminating ";" is not part
   of the value; however, WSP inside the value is significant.



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   Tags MUST be interpreted in a case-sensitive manner.  Values MUST be
   processed as case sensitive unless the specific tag description of
   semantics specifies case insensitivity.

   Tags with duplicate names MUST NOT occur within a single tag-list; if
   a tag name does occur more than once, the entire tag-list is invalid.

   Whitespace within a value MUST be retained unless explicitly excluded
   by the specific tag description.

   Tag=value pairs that represent the default value MAY be included to
   aid legibility.

   Unrecognized tags MUST be ignored.

   Tags that have an empty value are not the same as omitted tags.  An
   omitted tag is treated as having the default value; a tag with an
   empty value explicitly designates the empty string as the value.

3.3.  Signing and Verification Algorithms

   DKIM2 supports multiple digital signature algorithms.  Two algorithms
   are defined by this specification: RSA-SHA256 and Ed25519-SHA256.
   Signers SHOULD implement both RSA-SHA256 and Ed25519-SHA256.
   Verifiers MUST implement both RSA-SHA256 and Ed25519-SHA256.

3.3.1.  The RSA-SHA256 Signing Algorithm

   The RSA-SHA256 Signing Algorithm computes a message hash as described
   in Section 3.7 using SHA-256 (FIPS-180-4-2015) as the hash-alg.  That
   hash is then signed by the Signer using the RSA algorithm (defined in
   PKCS#1 version 1.5 [RFC8017]) as the crypt-alg and the Signer's
   private key.  The hash MUST NOT be truncated or converted into any
   form other than the native binary form before being signed.  The
   signing algorithm SHOULD use a public exponent of 65537.

   Signers MUST use RSA keys of at least 1024 bits.  Verifiers MUST be
   able to validate signatures with keys ranging from 1024 bits to 2048
   bits, and they MAY be able to validate signatures with larger keys.

3.3.2.  The Ed25519-SHA256 Signing Algorithm

   The Ed25519-SHA256 signing algorithm computes a message hash as
   defined in Section 3 of [RFC6376] using SHA-256 (FIPS-180-4-2015) as
   the hash-alg.  It signs the hash with the PureEdDSA variant Ed25519,
   as defined in Section 5.1 of [RFC8032].





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3.3.3.  Other Algorithms

   Other algorithms MAY be defined in the future.  Verifiers MUST ignore
   any signatures using algorithms that they do not implement.

3.4.  Canonicalization

   Some mail systems modify email in transit, potentially invalidating a
   signature.  DKIM2 provides a method of documenting such changes as
   they occur.  However, in order to reduce the necessity to document
   minor changes to header fields DKIM2 uses an algorithm for computing
   header hashes which permits minor changes.  This is identical to the
   "relaxed" algorithm of DKIM1 ([RFC6376]).  For the body no
   modifications are permitted (equivalent to the DKIM1 "relaxed"
   algorithm.

   Canonicalization simply prepares the email for presentation to the
   signing or verification algorithm.  It MUST NOT change the
   transmitted data in any way.  Canonicalization of header fields and
   body are described below.

   NOTE: This section assumes that the message is already in "network
   normal" format (text is ASCII encoded, lines are separated with CRLF
   characters, etc.).  See also Section 5.4 for information about
   normalizing the message.

3.4.1.  The Header Canonicalization Algorithm

   All header fields present in a message are signed, with the exception
   of Trace Header Fields.  "Trace headers" are described in [RFC5321]
   and [RFC5322] and documented in an IANA registry established by
   [RFC3864].  At present the only header fields designated as "Trace"
   are Received and Return-path.

   The header canonicalization algorithm MUST apply the following steps
   in order:

   *  Ignore any trace header fields that are present.

   *  Convert all header field names (not the header field values) to
      lowercase.  For example, convert "SUBJect: AbC" to "subject: AbC".

   *  Unfold all header field continuation lines as described in
      [RFC5322]; in particular, lines with terminators embedded in
      continued header field values (that is, CRLF sequences followed by
      WSP) MUST be interpreted without the CRLF.  Implementations MUST
      NOT remove the CRLF at the end of the header field value.




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   *  Convert all sequences of one or more WSP characters to a single SP
      character.  WSP characters here include those before and after a
      line folding boundary.

   *  Delete all WSP characters at the end of each unfolded header field
      value.

   *  Delete any WSP characters remaining before and after the colon
      separating the header field name from the header field value.  The
      colon separator MUST be retained.

   *  Place the header fields in alphabetical order by the header field
      name, except for any header fields that start "DKIM2" which are
      placed last.

   *  If there is more than one header with the same header field name
      then the header fields are placed in the order in which they occur
      in the email header.  It is sometimes suggested that some MTAs re-
      order header fields after they receive an email, if they do then
      it is their responsibility to recover the original order of any
      header fields with identical header field names (that are part of
      a signature calculation) before verifying an existing signature or
      passing a previously signed message to another MTA that is going
      to wish to verify a signature.

   *  The DKIM2 header fields are placed at the end of the list of
      header fields to be signed, ordered first by their "i=" value (in
      ascending numerical order) and then by their header field name.

   Note that the special rules for ordering DKIM2 header fields are
   intended to assist systems that wish to pre-calculate a hash value
   for all the other header fields and then finish off with the actual
   DKIM2 headers that they will be adding.

3.4.2.  The Body Canonicalization Algorithm

   The body canonicalization algorithm MUST apply the following steps in
   order:

   *  Ignore all whitespace at the end of lines.  Implementations MUST
      NOT remove the CRLF at the end of the line.

   *  Reduce all sequences of WSP within a line to a single SP
      character.







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   *  Ignore all empty lines at the end of the message body.  An empty
      line is a line of zero length after removal of the line
      terminator.  If there is no body or no trailing CRLF on the
      message body, a CRLF is added.

   Note that a completely empty or missing body is canonicalized as a
   single "CRLF"; that is, the canonicalized length will be 2 octets.

3.5.  The DKIM2-Signature Header Field

   The signature of the email is stored in the DKIM2-Signature header
   field.  This header field contains all of the signature and key-
   fetching data.  The DKIM2-Signature value is a tag-list as described
   in Section 3.2.

   The DKIM2-Signature header field SHOULD be treated as though it were
   a trace header field as defined in Section 3.6 of [RFC5322] and hence
   SHOULD NOT be reordered and SHOULD be prepended to the message.

   The DKIM2-Signature header field being created or verified is always
   included in the signature calculation, after the rest of the header
   fields being signed; however, when calculating or verifying the
   signature, the value of the "b=" tag (signature value) of that DKIM-
   Signature header field MUST be treated as though it were an empty
   string.  Unknown tags in the DKIM2-Signature header field MUST be
   included in the signature calculation.

   Tags on the DKIM2-Signature header field along with their type,
   encoding and requirement status are shown below.  It will be noted
   that we have not included a version number.  Experience from IMF
   onwards shows that it is essentially impossible to change version
   numbers.  If it becomes necessary to change DKIM2 in the sort of
   incompatible way that a v=2 / v=3 version number would support, it is
   expected that header fields will be labelled as DKIM3 instead.

   i= The sequence number of the DKIM2-Signature header field.

   plain-text unsigned decimal integer; REQUIRED

   The originator of a message uses the value 1.  Further
   DKIM2-Signature header fields are added with a value one more than
   the current highest numbered DKIM2-Signature header field.  Gaps in
   the numbering MUST be treated as making the whole message unsigned.

   ABNF:

   sig-i-tag    = %x69 [FWS] "=" [FWS] 1*2DIGIT




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   n= Nonce value

   plain-text unsigned decimal integer; OPTIONAL

   This value, if present, has a meaning to the creator of the signature
   but MUST NOT be assumed to have any meaning to any other entity.  It
   MAY be used as an index into a database to assist in handling
   Delivery Status Notifications or for any other purpose.

   ABNF:

   sig-n-tag    = %x6e [FWS] "=" [FWS] 1*(DIGIT)

   t= Signature Timestamp

   plain-text unsigned decimal integer; REQUIRED

   The time that this header field was created.  The format is the
   number of seconds since 1970-01-01T00:00:00 UTC.  This value is not
   constrained to fit into a 31- or 32-bit integer.  Implementations
   SHOULD be prepared to handle values up to at least 10^12 (until
   approximately AD 200,000; this fits into 40 bits).  Implementations
   MAY ignore signatures that have a timestamp in the future.
   Implementations MAY ignore signatures that are more than 14 days old.

   ABNF:

   sig-t-tag    = %x74 [FWS] "=" [FWS] 1*12DIGIT

   mf= The [RFC5321] MAIL FROM value to be used when this message is
   transmitted over an SMTP link from the signing MTA.

   plain-text; REQUIRED

   Note that MAIL FROM may be just "<>", for example for a Delivery
   Status Notification.

   ABNF:

   sig-mf-tag = %x6d %65 [FWS] "="
                [FWS] "<" [ local-part "@" domain-name ] ">"

   rt= The [RFC5321] RCPT TO value(s) to be used when this message is
   transmitted over an SMTP link from the signing MTA.

   plain-text; REQUIRED





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   When a message is intended for more than one recipient then this
   field MAY include all of the recipients so that a single copy of the
   email MAY be sent to all of the recipients in a single SMTP
   transaction.  Note that if "bcc:" recipients are involved then in
   order to meet the requirements of [RFC5322] Section 3.6.3 each bcc
   recipient MUST be sent a message with just their email address
   appearing in this tag.

   ABNF:

   sig-rt-tag = %72 %x74 [FWS] "="
                1*( [FWS] "<" local-part "@" domain-name ">" )

   d= The domain associated with this signature.

   plain-text; REQUIRED

   This domain is used to form the query for the public key.  The domain
   MUST be a valid DNS name under which the DKIM2 key record is
   published.  Internationalized domain names MUST be encoded as
   A-labels, as described in Section 2.3 of [RFC5890].

   The domain in the d= tag MUST exactly match the rightmost labels of
   the domain-name part of the mf= tag.  That is to say, the domain-name
   of the mf= tag MUST either match the d= domain exactly or be a sub-
   domain of d= domain.

   ABNF:

   sig-d-tag   = %x64 [FWS] "=" [FWS] domain-name
   domain-name = sub-domain 1*("." sub-domain)
                            ; from [RFC5321] Domain,
                            ; excluding address-literal

   s1= The selector subdividing the namespace for the "d=" (domain) tag.

   plain-text; REQUIRED

   Internationalized selector names MUST be encoded as A-labels, as
   described in Section 2.3 of [RFC5890].

   ABNF:

   sig-s-tag = %x73 %x31 [FWS] "=" [FWS] selector

   a1= The algorithm used to generate the signature.

   plain-text; REQUIRED



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   Verifiers MUST support "rsa-sha256" and "ed25519"; See Section 3.3
   for a description of the algorithms.

   ABNF:

   sig-a-tag     = %x61 %x31 [FWS] "=" [FWS] sig-a-tag-alg
   sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h
   sig-a-tag-k   = "rsa" / "ed25519" / x-sig-a-tag-k
   sig-a-tag-h   = "sha256" / x-sig-a-tag-h
   x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT)
                            ; for later extension
   x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT)
                            ; for later extension

   b1= The signature data.

   base64; REQUIRED

   Whitespace is ignored in this value and MUST be ignored when
   reassembling the original signature.  In particular, the signing
   process can safely insert FWS in this value in arbitrary places to
   conform to line-length limits.  See "Signer Actions" (Section 5) for
   how the signature is computed.

   ABNF:

   sig-b-tag      = %x62 %x31 [FWS] "=" [FWS] sig-b-tag-data
   sig-b-tag-data = base64string

   bh1= The hash of the canonicalized body part of the message.

   base64; REQUIRED

   Whitespace is ignored in this value and MUST be ignored when
   reassembling the original signature.  In particular, the signing
   process can safely insert FWS in this value in arbitrary places to
   conform to line-length limits.  See Section 3.7 for how the body hash
   is computed.

   ABNF:

   sig-bh-tag      = %x62 %x68 %x31 [FWS] "=" [FWS] sig-bh-tag-data
   sig-bh-tag-data = base64string

   s2=, a2= b2= bh2= Further signatures (equivalent to s1, a1, b2 and
   bh1)

   plain text / base64; OPTIONAL



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   To provide for algorithmic dexterity a second signature, using a
   different algorithm MAY be supplied.  A verifier MUST check all
   signatures that it understands and SHOULD treat any failure as
   invalidating all signatures.

   f= Flags

   plain text; OPTIONAL

   Flags serve two purposes; they either report what has been done to
   the message by the system creating the DKIM2-Signature or they make a
   request to systems that handle the mail thereafter.  Flags are
   separated by commas, and optional white-space allows systems to add
   several flags without creating long lines.

   If a flag value is not recognised it MUST be ignored.

   The flag values that report things are:

   "exploded": this message (identified by its unique header hash value
   (recorded in b1= and perhaps b2=)) is being sent to more than one
   email address.  An MTA which receives a message MAY use this
   information to help it distinguish between malicious "DKIM replay"
   and legitimate activity performed by mailing lists.

   "modifiedbody": the body of the message has been altered in some way.
   A corresponding DKIM2-Delta-Body MUST be present with the same i=
   value as defined in [ALGEBRA].  This flag MUST NOT be present when
   i=1;

   "modifiedheader": the header of the message has been altered in some
   way.  A corresponding DKIM2-Delta-Header must be present with the
   same i= value as defined in [ALGEBRA].  This flag MUST NOT be present
   when i=1;

   The flags values that make requests are:

   "donotexplode": this signer requests that the message not be sent to
   more than one recipient.  A system that, by local policy, ignores
   this request MUST NOT allow any of the copies it creates to be
   forwarded on to any MTA outside its control.

   "donotmodify": this signer requests that the message not be modified
   from the form in which it is sent.  A system that, by local policy,
   ignores this request MUST NOT allow the message to be forwarded on to
   any MTA outside its control.





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   "feedback": this signer requests feedback about how this message is
   handled during delivery and thereafter.  This document does not
   describe what such feedback might be or where it might be delivered.
   If this flag is absent then feedback is explicitly not required.

   ABNF:

 sig-f-tag = %x66 [FWS] "=" [FWS] sig-f-data *("," [FWS] sig-f-tag-data)
 sig-f-tag-data   = "modifiedbody" | "modifiedheader" | "exploded" |
                    "donotmodify" | "donotexplode" | "feedback"
 x-sig-f-tag-data = ALPHA *(ALPHA / DIGIT)
                          ; for later extension

3.6.  Key Management

   Signature applications require some level of assurance that the a
   public key is associated with the claimed Signer.  DKIM2 does this by
   fetching the key from the DNS for the domain specified in the d=
   field.  These keys are no different, and are stored in the same
   locations as those for DKIM1 ([RFC6376]).

   All DKIM keys are stored in a subdomain named "_domainkey".  Given a
   DKIM2-Signature field with a "d=" tag of "example.com" and an "s1="
   tag of "foo.bar", the DNS query will be for
   "foo.bar._domainkey.example.com".

3.7.  Computing the Message Hashes

   Both signing and verifying message signatures start with a step of
   computing two cryptographic hashes over the message.  Signers will
   choose the parameters of the signature as described in "Signer
   Actions" (Section 5); Verifiers will use the parameters specified in
   the DKIM2-Signature header field being verified.  In the following
   discussion, the names of the tags in the DKIM2-Signature header field
   that either exists (when verifying) or will be created (when signing)
   are used.  Note that canonicalization (Section 3.4) is only used to
   prepare the email for signing or verifying; it does not affect the
   transmitted email in any way.

   For clarity this section will discuss the first pair of signatures
   (s1=, a1=, h1=, bh1).  If a second pair (s2=, a2=, h2=, bh2=) is
   present then that is calculated or verified in an identical manner
   except using the "2" values rather than the "1" values.

   The Signer/Verifier MUST compute two hashes: one over the body of the
   message and one over relevant header fields of the message.





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   Signers MUST compute them in the order shown.  Verifiers MAY compute
   them in any order convenient to the Verifier, provided that the
   result is semantically identical to the semantics that would be the
   case had they been computed in this order.

   In hash step 1, the Signer/Verifier MUST hash the message body,
   canonicalized using the body canonicalization algorithm.  That hash
   value is then converted to base64 form and inserted into (Signers) or
   compared to (Verifiers) the "bh1=" tag of the DKIM- Signature header
   field.

   In hash step 2, the Signer/Verifier MUST pass header fields to the
   hash algorithm as specified in Section 3.4.1.

   After these header fields add any DKIM2-Delta-Header header field (as
   specified in [ALGEBRA] for the current i= value.  This header field
   has its CRLF included in the hash in the normal way.

   Finally add a canonicalized copy of the DKIM2-Signature header field
   that exists (verifying) or will be inserted (signing) in the message,
   with the value of the "b=" tag (including all surrounding whitespace)
   deleted (i.e., treated as the empty string) without a trailing CRLF.

   When calculating the hash on messages that will be transmitted using
   base64 or quoted-printable encoding, Signers MUST compute the hash
   after the encoding.  Likewise, the Verifier MUST incorporate the
   values into the hash before decoding the base64 or quoted-printable
   text.  However, the hash MUST be computed before transport-level
   encodings such as SMTP "dot-stuffing" (the modification of lines
   beginning with a "." to avoid confusion with the SMTP end-of-message
   marker, as specified in [RFC5321]).

   With the exception of the canonicalization procedure described in
   Section 3.4, the DKIM2 signing process treats the body of messages as
   simply a string of octets.  DKIM2 messages MAY be either in plain-
   text or in MIME format; no special treatment is afforded to MIME
   content.  Message attachments in MIME format MUST be included in the
   content that is signed.

3.8.  Input Requirements

   A message that is not compliant with [RFC5322], [RFC2045], and
   [RFC2047] can be subject to attempts by intermediaries to correct or
   interpret such content.  See Section 8 of [RFC6409] for examples of
   changes that are commonly made.  Such "corrections" may invalidate
   DKIM2 signatures or have other undesirable effects, including some
   that involve changes to the way a message is presented to an end
   user.



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   Accordingly, DKIM2's design is predicated on valid input.  Therefore,
   Signers and Verifiers SHOULD take reasonable steps to ensure that the
   messages they are processing are valid according to [RFC5322],
   [RFC2045], and any other relevant message format standards.

3.9.  Output Requirements

   The evaluation of each signature ends in one of three states, which
   this document refers to as follows:

   SUCCESS: a successful verification

   PERMFAIL: a permanent, non-recoverable error such as a signature
   verification failure

   TEMPFAIL: a temporary, recoverable error such as a DNS query timeout

   For each signature that verifies successfully or produces a TEMPFAIL
   result, output of the DKIM2 algorithm MUST include the set of:

   *  The domain name, taken from the "d=" signature tag; and

   *  The result of the verification attempt for that signature.

   The output MAY include other signature properties or result meta-
   data, including PERMFAILed or otherwise ignored signatures, for use
   by modules that consume those results.

   See Section 6.1 for discussion of signature validation result codes.

4.  Semantics of Multiple Signatures

   In DKIM2 a new signature (with an incremented i= value) is added by
   every MTA that handles a message.  However, if these MTAs are
   handling email within an organization then it is only necessary for
   the MTA that sends the email outside of that organization to apply a
   DKIM2 signature.

   Verifiers may check the entire chain of signatures to establish that
   there is a valid chain from start to finish; however where the chain
   shows that the email has not been modified during its travels it will
   generally be sufficient to check only the first and last signature.

   When there is evidence of "DKIM replay" inspection of the entire
   chain will enable the location of the replay to be determined and
   appropriate action can then be taken.

   Further information about multiple signatures can be found in TBA.



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5.  Signer Actions

   The following steps are performed in order by Signers.

5.1.  Document modifications made to a message

   MTAs that accept a DKIM2 signed message and send it onward to another
   MTA MUST record the changes that they have made to the message.  A
   scheme for generating appropriate DKIM2-Delta-Header header fields to
   describe such modifications can be found in [ALGEBRA].

   Note in particular that adding header fields must be documented, with
   the exception of trace header fields (such as Received:) that are not
   signed.

   Failure to record modifications will mean that an MTA that
   subsequently handles the message SHOULD detect this and this MAY lead
   to the message being rejected.

5.2.  Determine what [RFC5321] "envelope" will be used for the message

   The DKIM2-Signature field contains mf= and rt= values, so the MAIL
   FROM and RCPT TO values that will be used when the message is
   transmitted must be available to (or deducible by) a Signer or
   Verifier.

   When the message being signed already has a DKIM2-Signature header
   field (i.e. it has already been transmitted at least once) then the
   mf= of the message to be signed MUST match with an entry in the rt=
   of currently highest numbered (i=) DKIM2-Signature header field.  If
   this will not be the case then a Signer MUST generate an extra
   DKIM2-Signature field that causes values to match, noting that the
   creation of this extra header field will require the Signer to have
   access to the DKIM2 key value associated with a domain in the rt=
   entry.

5.3.  Select a Private Key and Corresponding Selector Information

   This specification does not define the basis by which a Signer should
   choose which private key and selector information to use.  Currently,
   all selectors are equal as far as this specification is concerned, so
   the decision should largely be a matter of administrative
   convenience.  Distribution and management of private keys is also
   outside the scope of this document.







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5.4.  Normalize the Message to Prevent Transport Conversions

   Some messages, particularly those using 8-bit characters, are subject
   to modification during transit, notably conversion to 7-bit form.
   Such conversions will break DKIM2 signatures.  Hence the message
   SHOULD be converted to only contain 7-bit characters, for example by
   employing a suitable MIME content-transfer encoding such as quoted-
   printable or base64 as described in [RFC2045].  The way in which this
   is achieved is outside the scope of this specification.

   If the message contains local encoding that will be modified before
   transmission, that modification to canonical [RFC5322] form MUST be
   done before signing.  In particular, bare CR or LF characters (used
   by some systems as a local line separator convention) MUST be
   converted to the SMTP-standard CRLF sequence before the message is
   signed.  Any conversion of this sort SHOULD be applied to the message
   actually sent to the recipient(s), not just to the version presented
   to the signing algorithm.

   More generally, the Signer MUST sign the message as it is expected to
   be received by the Verifier rather than in some local or internal
   form.

5.5.  Compute the Message Hash and Signature{#signer-compute}

   The Signer MUST compute the message hash as described in Section 3.7
   and then sign it using the selected public key algorithm.  This will
   result in a DKIM2-Signature header field that will include the body
   hash and a signature of the header hash, where that header includes
   the DKIM2-Signature header field itself.

   Entities such as mailing list managers that implement DKIM2 and that
   modify the message or a header field (for example, inserting
   unsubscribe information) before retransmitting the message SHOULD
   check any existing signature on input and MUST make such
   modifications before re-signing the message.

5.6.  Insert the DKIM2-Signature Header Field

   Finally, the Signer MUST insert the DKIM2-Signature header field
   created in the previous step prior to transmitting the email.  The
   DKIM2-Signature header field MUST be the same as used to compute the
   hash as described above, except that the value of the "b=" tag MUST
   be the appropriately signed hash computed in the previous step,
   signed using the algorithm specified in the "a=" tag of the DKIM2-
   Signature header field and using the private key corresponding to the
   selector given in the "s=" tag of the DKIM2-Signature header field,
   as chosen above in Section 5.3}.



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   The DKIM2-Signature header field MUST be inserted before any other
   DKIM2-Signature fields in the header block.

   INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this is
   to insert the DKIM2-Signature header field at the beginning of the
   header block before any existing Received header fields.  This is
   consistent with treating DKIM2-Signature as a trace header field.

6.  Verifier Actions

   A border or intermediate MTA MAY verify the message signature(s).  An
   MTA that has performed verification MAY communicate the result of
   that verification by adding a verification header field to incoming
   messages.  This simplifies things considerably for the user, who can
   now use an existing mail user agent.  Most MUAs have the ability to
   filter messages based on message header fields or content; these
   filters would be used to implement whatever policy the user wishes
   with respect to unsigned mail.

   A verifying MTA MAY implement a policy with respect to unverifiable
   mail, regardless of whether or not it applies the verification header
   field to signed messages.

   Verifiers MUST produce a result that is semantically equivalent to
   applying the steps listed in Section 6.1, Section 6.1.1, and
   Section 6.1.2 in order.  In practice, several of these steps can be
   performed in parallel in order to improve performance.

6.1.  Extract Signatures from the Message

   A Verifier SHOULD check the most recently applied (highest numbered
   i= value) DKIM2-Signature before accepting an email.  If this check
   does not pass then a Delivery Status Notification for the email MUST
   NOT be generated thereafter -- hence the best strategy, if the email
   is not wanted, is to reject it (with a 5xx error code) whilst the
   relevant SMTP conversation is till ongoing.

   A Verifier that wishes to ascertain whether or not a message has been
   modified since it was first created need only check the first (i=1)
   DKIM2-Signature.

   If the message has been modified since it's original creation then
   the DKIM2-Modification header fields (see [ALGEBRA]) will enable a
   Verifier to determine whether or not all the changes made are
   correctly recorded.






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   For each signature to be validated, the following steps should be
   performed in such a manner as to produce a result that is
   semantically equivalent to performing them in the indicated order.

   Where the status is TEMPFAIL then it may be possible for the Verifier
   to determine the validity of the signature at a later time.  If the
   SMTP conversation is still ongoing then a 4xx error code SHOULD be
   used to allow the sending MTA to understand the situation.

6.1.1.  Validate the Signature Header Field

   Implementers MUST meticulously validate the format and values in the
   DKIM2-Signature header field; any inconsistency or unexpected values
   MUST cause the header field to be completely ignored and the Verifier
   to return PERMFAIL (signature syntax error).  Being "liberal in what
   you accept" is definitely a bad strategy in this security context.

   Note, however, that this does not include the existence of unknown
   tags in a DKIM2-Signature header field, which are explicitly
   permitted.

   Verifiers MAY return PERMFAIL (signature expired) if it is more than
   14 days since the timestamp recorded in the "t=" tag.

6.1.2.  Get the Public Key

   The public key for a signature is needed to complete the verification
   process.  Details of key management and representation are described
   in Section 3.6.  The Verifier MUST validate the key record and MUST
   ignore any public key records that are malformed.

   NOTE: The use of a wildcard TXT RR that covers a queried DKIM domain
   name will produce a response to a DKIM query that is unlikely to be a
   valid DKIM key record.  This problem is not specific to DKIM and
   applies to many other types of queries.  Client software that
   processes DNS responses needs to take this problem into account.

   When validating a message, a Verifier MUST perform the following
   steps in a manner that is semantically the same as performing them in
   the order indicated; in some cases, the implementation may
   parallelize or reorder these steps, as long as the semantics remain
   unchanged:

   1.  The Verifier retrieves the public key as described in Section 5.3
       using the algorithm in the "a=" tag, the domain from the "d="
       tag, and the selector from the "s=" tag.





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   2.  If the query for the public key fails to respond, the Verifier
       MAY seek a later verification attempt by returning TEMPFAIL (key
       unavailable).

   3.  If the query for the public key fails because the corresponding
       key record does not exist, the Verifier MUST return PERMFAIL (no
       key for signature).

   4.  If the query for the public key returns multiple key records, the
       Verifier can choose one of the key records or may cycle through
       the key records, performing the remainder of these steps on each
       record at the discretion of the implementer.  The order of the
       key records is unspecified.  If the Verifier chooses to cycle
       through the key records, then the "return ..." wording in the
       remainder of this section means "try the next key record, if any;
       if none, return to try another signature in the usual way".

   5.  If the result returned from the query does not adhere to the
       format defined in the DKIM key specification [DKIMKEYS], the
       Verifier MUST ignore the key record and return PERMFAIL (key
       syntax error).  Verifiers are urged to validate the syntax of key
       records carefully to avoid attempted attacks.  In particular, the
       Verifier MUST ignore keys with a version code ("v=" tag) that
       they do not implement.

   6.  If the public key data (the "p=" tag) is empty, then this key has
       been revoked and the Verifier MUST treat this as a failed
       signature check and return PERMFAIL (key revoked).  There is no
       defined semantic difference between a key that has been revoked
       and a key record that has been removed.

   7.  If the public key data is not suitable for use with the algorithm
       and key types defined by the "a=" and "k=" tags in the DKIM-
       Signature header field, the Verifier MAY immediately return
       PERMFAIL (inappropriate key algorithm).  However, these fields
       are now deprecated so DKIM2 implementations MAY ignore them
       altogether.

   8.  If the "h=" tag exists in the public key record and the hash
       algorithm implied by the "a=" tag in the DKIM2-Signature header
       field is not included in the contents of the "h=" tag, the
       Verifier MUST ignore the key record and return PERMFAIL
       (inappropriate hash algorithm).

6.1.3.  Compute the Verification

   Given a Signer and a public key, verifying a signature consists of
   actions semantically equivalent to the following steps.



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   1.  Prepare a canonicalized version of the message as is described in
       Section 3.7 (note that this canonicalized version does not
       actually replace the original content).

   2.  Based on the algorithm indicated in the "a=" tag, compute the
       message hashes from the canonical copy as described in
       Section 3.7.

   3.  Verify that the hash of the canonicalized message body computed
       in the previous step matches the hash value conveyed in the "bh="
       tag.  If the hash does not match, the Verifier SHOULD ignore the
       signature and return PERMFAIL (body hash did not verify).

   4.  Using the signature conveyed in the "b=" tag, verify the
       signature against the header hash using the mechanism appropriate
       for the public key algorithm described in the "a=" tag.  If the
       signature does not validate, the Verifier SHOULD ignore the
       signature and return PERMFAIL (signature did not verify).

   5.  Otherwise, the signature has correctly verified.

6.2.  Communicate Verification Results

   Verifiers wishing to communicate the results of verification to other
   parts of the mail system may do so in whatever manner they see fit.
   For example, implementations might choose to add an email header
   field to the message before passing it on.  Any such header field
   SHOULD be inserted before any existing DKIM2-Signature or pre-
   existing authentication status header fields in the header field
   block.  The Authentication-Results: header field ([RFC8601]) MAY be
   used for this purpose.

6.3.  Interpret Results/Apply Local Policy

   It is beyond the scope of this specification to describe what actions
   the recipient of an email performs, but mail carrying valid DKIM2
   signatures gives the recipient opportunities that unauthenticated
   email would not.  Specifically, an authenticated email provides
   predictable information by which other decisions can reliably be
   managed, such as trust and reputation.  Conversely, it is hard to
   assign trust or reputation to unauthenticated email.










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   In general, modules that consume DKIM2 verification results SHOULD
   NOT determine message acceptability based solely on a lack of any
   signature or on an unverifiable signature; such rejection would cause
   severe interoperability problems.  If an MTA does wish to reject such
   messages during an SMTP session (for example, when communicating with
   a peer who, by prior agreement, agrees to only send signed messages),
   and a signature is missing or does not verify, the handling MTA
   SHOULD use a 550/5.7.x reply code.

   Where the Verifier is integrated within the MTA and it is not
   possible to fetch the public key, perhaps because the key server is
   not available, a temporary failure message MAY be generated using a
   451/4.7.5 reply code, such as:

   451 4.7.5 Unable to verify signature - key server unavailable

   Temporary failures such as inability to access the key server or
   other external service are the only conditions that SHOULD use a 4xx
   SMTP reply code.  In particular, cryptographic signature verification
   failures MUST NOT provoke 4xx SMTP replies.

   If the email cannot be verified, then it SHOULD be treated the same
   as all unverified email, regardless of whether or not it looks like
   it was signed.

7.  IANA Considerations

   TBA

8.  Security Considerations

   TBA

9.  References

9.1.  Normative References

   [ALGEBRA]  Gondwana, B., "A method for describing changes made to an
              email", Work in Progress, Internet-Draft, draft-gondwana-
              dkim2-modification-alegbra-02, 18 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-gondwana-
              dkim2-modification-alegbra-02>.

   [DKIMKEYS] Chuang, W., "Domain Name Specification for DKIM2", Work in
              Progress, Internet-Draft, draft-chuang-dkim2-dns-02, 7
              July 2025, <https://datatracker.ietf.org/doc/html/draft-
              chuang-dkim2-dns-02>.




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   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/rfc/rfc1034>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/rfc/rfc2045>.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <https://www.rfc-editor.org/rfc/rfc2047>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/rfc/rfc3629>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/rfc/rfc3864>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/rfc/rfc5234>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/rfc/rfc5321>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/rfc/rfc5322>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/rfc/rfc5890>.






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   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/rfc/rfc6376>.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
              <https://www.rfc-editor.org/rfc/rfc6409>.

   [RFC8601]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 8601,
              DOI 10.17487/RFC8601, May 2019,
              <https://www.rfc-editor.org/rfc/rfc8601>.

9.2.  Informative References

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009,
              <https://www.rfc-editor.org/rfc/rfc5598>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/rfc/rfc8017>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/rfc/rfc8032>.

Appendix A.  Changes from Earlier Versions

   [[This section to be removed by RFC Editor]]

Authors' Addresses

   Richard Clayton
   Yahoo
   Email: rclayton@yahooinc.com


   Wei Chuang
   Google
   Email: weihaw@google.com







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   Bron Gondwana
   Fastmail Pty Ltd
   Level 2, 114 William Street
   3000
   Australia
   Phone: +61 457 416 436
   Email: brong@fastmailteam.com












































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