DMARC Working Group                                       F. Martin, Ed.
Internet-Draft                                                  LinkedIn
Intended status: Informational                              E. Lear, Ed.
Expires: September 21, October 30, 2015                             Cisco Systems GmbH
                                                         T. Draegen, Ed.
                                              Eudaemonic Development LLC
                                                          E. Zwicky, Ed.
                                                          March 20,
                                                          April 28, 2015

     Interoperability Issues Between DMARC and Indirect Email Flows


   DMARC introduces a mechanism for expressing domain-level policies and
   preferences for email message validation, disposition, and reporting.
   The DMARC mechanism can encounter interoperability issues when
   messages originate do not flow directly from third party sources, are modified in transit,
   or are forwarded enroute the author's administrative domain
   to their the final destination. recipients.  Collectively these email flows are referred
   to as indirect email flows.  This document describes interoperability
   issues between DMARC and indirect email flows.  Possible methods for
   addressing interoperability issues are presented.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Document Conventions  . . . . . . . . . . . . . . . . . .   3
   2.  Causes of Interoperability Issues . . . . . . . . . . . . . .   3
     2.1.  Identifier Alignment  . . . . . . . . . . . . . . . . . .   4
     2.2.  Message Forwarding  . . . . . . . . . . . . . . . . . . .   5
     2.3.  Message Modification  . . . . . . . . . . . . . . . . . .   5
   3.  Internet Mail Architecture, DMARC, and Indirect Email Flows .   5   6
     3.1.  Message Handling System . . . . . . . . . . . . . . . . .   5   6
       3.1.1.  Message Submission Agents . . . . . . . . . . . . . .   6
       3.1.2.  Message Transfer Agents . . . . . . . . . . . . . . .   7  Message Encoding  . . . . . . . . . . . . . . . .   7  Header Standardization  . . . . . . . . . . . . .   7  Email Address Internationalization  . . . . . . .   7   8
       3.1.3.  Message Delivery Agents . . . . . . . . . . . . . . .   8
     3.2.  Mediators . . . . . . . . . . . . . . . . . . . . . . . .   8   9
       3.2.1.  Alias . . . . . . . . . . . . . . . . . . . . . . . .   8   9
       3.2.2.  ReSenders . . . . . . . . . . . . . . . . . . . . . .   9  10
       3.2.3.  Mailing Lists . . . . . . . . . . . . . . . . . . . .  10
       3.2.4.  Gateways  . . . . . . . . . . . . . . . . . . . . . .  10  11
       3.2.5.  Boundary Filters  . . . . . . . . . . . . . . . . . .  11
     3.3.  Combinations  . . . . . . . . . . . . . . . . . . . . . .  11  12
   4.  Possible Solutions to Mitigations of Interoperability Issues . . . . . . . .  12
     4.1.  Identifier Alignment  . . . . . .  Mitigations in Current Use  . . . . . . . . . . . .  12
     4.2.  Message Modification . . .  13
       4.1.1.  Mitigations for Senders . . . . . . . . . . . . . . .  13
     4.3.  Message Forwarding  . . . .  Identifier Alignment  . . . . . . . . . . . . . .  13  Message Modification  .  14
       4.3.1.  Original-Authentication-Results . . . . . . . . . . .  14
     4.4.  Message Handling Services . .  13
       4.1.2.  Mitigations for Receivers . . . . . . . . . . . . . .  14
       4.4.1.  Message Transfer Agents .  Identifier Alignment  . . . . . . . . . . . . . .  14  Encoding  . . .  Policy Override . . . . . . . . . . . . . . . . .  14  Filters  Email Address Internationalization  . . . . . . .  14
       4.1.3.  Mitigations for ReSenders . . . . . . . . . . . . . .  14  Email Address Internationalization  . .  Changes to the RFC5322.From . . . . .  15
     4.5.  Mediators . . . . . .  14  Avoiding Message Modification . . . . . . . . . .  15  Mailing Lists . . . . . . . .  15
       4.5.1.  Mailing Lists . . . . . . . . . .  15
     4.2.  Proposed and In-Progress Mitigations  . . . . . . . . . .  15
     4.6.  16
       4.2.1.  Getting More Radical: Requiring New Communication
               Paths Between MUA and the Message Store MUAs  . . . . . . . . . . . .  16 . . . . .  17
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16  17
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16  17
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17  18
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  17  18
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18  20

1.  Introduction

   DMARC [RFC7489] introduces a mechanism for expressing domain-level
   policies and preferences for message validation, disposition, and
   reporting.  DMARC is used to combat exact-domain phishing, to gain
   visibility into email infrastructure, and to provide email egress
   controls.  Due to wide adoption, the impact of DMARC-based email
   rejection policies on both direct and indirect email flows can be
   significant.  The DMARC mechanism can encounter several different types
   of interoperability issues due to third-party message sourcing,
   message transformation or rerouting.  These cases

   Cases in which mail email does not go flow directly from the author's
   administrative domain to the recipients are known collectively referred to
   in this document as indirect email flows.

   The next section describes interoperability issues between DMARC  Due to existing and
   increasing adoption of DMARC, the impact of DMARC-based email
   rejection policies on both direct and indirect email flows.  These flows can be

   Several known causes of interoperability issues are first described in the
   context of configuration behavior that DMARC requires from underlying
   authentication technology, and then described as they appear in
   context presented,
   followed by a description of components within the Internet Mail
   Architecture [RFC5598]. [RFC5598] where interoperability issues can arise.

   Lastly, known and possible methods for addressing interoperability
   issues are presented.  There are often multiple ways to address any
   given interoperability issue.  While this document strives to be
   comprehensive in its review, it should not be treated as complete.

1.1.  Document Conventions

   Notation regarding structured fields is taken from [RFC5598].

   Organizational Domain and Authenticated Identifiers are specified in
   DMARC [RFC7489].

2.  Causes of Interoperability Issues

   What do we mean by "interoperability issues"?  We say that

   Interoperability issues between DMARC
   introduces interoperability issues or problems, and indirect email flows arise
   when conformance to the DMARC specification leads an implementation
   to reject a message apply DMARC based policy to messages that is are both compliant with
   the architecture as specified in [RFC5598] and would have been viewed as legitimate in the eyes of
   by the intended recipient.  Therefore, we can already conclude that DMARC
   poses no interoperability problems when  To be clear, this document does not
   address emails considered legitimate messages properly
   validate through its specified processes. by the intended recipient but
   which are not conformant to other email specifications.  The rest of
   this section
   delves into how legitimate messages may get rejected. describes several conceptual causes of interoperability

2.1.  Identifier Alignment

   A fundamental aspect of message source validation is understanding
   what defines the source that is validated.  Each of the underlying
   mechanisms that

   DMARC uses (DKIM relies on DKIM [RFC6376] and SPF [RFC7208]) takes a
   different approach.  Therefore, the [RFC7208] to perform message
   source validation.  The DMARC [RFC7489] mechanism
   attempts refers to source
   domains that are validated by DKIM or SPF as Authenticated
   Identifiers.  DMARC requires an Authenticated Identifier to be
   relevant to predictably specify the domain of found in the originator that
   will be used for its purposes (reporting/message disposition). message's RFC5322.From header
   field [RFC5322].  This
   step relevancy is referred to as Identifier

   Identifier Alignment can be strict, where the domains exactly match
   each others, or relaxed where the domains are part of the same
   Organizational Domain.  There are, in general, the same
   interoperability issues between strict and relaxed alignment, however
   in strict mode the possible solutions are more constrained when
   possible.  This Document mainly implies relaxed Identifier Alignment.

   DKIM provides a cryptographic means for a domain to be associated
   with a particular message.  DKIM does not make any constraints on
   what domains may or must present this association.  However, for a
   DKIM identifier to align in DMARC, the signing domain of a valid
   signature must be part of the same Organizational Domain as the
   domain in the RFC5322.From header field [RFC5322], and the signature must be valid. [RFC5322].

   In addition, DKIM allows for the possibility of multiple valid
   signatures.  The DMARC mechanism will process Authenticated
   Identifiers that are based on DKIM signatures until an aligned
   Authenticated Identifier is found (if any).  However, operational
   experience has shown that some implementations have difficulty
   processing multiple signatures.  The impact on DMARC processing is
   clear: if an implementation implementations that cannot process multiple DKIM signatures
   may lead erroneously apply DMARC based policy to perfectly valid messages being flagged as not
   authentic. otherwise legitimate

   SPF can provides two Authenticated Identifiers the first one is
   RFC7208.HELO [RFC7208] based on RFC5321.HELO/EHLO two different
   SPF identities: RFC7208.HELO [RFC7208] and the second one
   is RFC7208.MAILFROM [RFC7208]  based on the RFC5321.MailFrom
   [RFC5321] domain or, if the RFC5321.MailFrom address is absent (as in
   the case of "bounces"), on the domain found in the HELO/EHLO SMTP
   command.  Local policies, as well as
   [RFC7208].  DMARC often uses only use the RFC7208.MAILFROM identifier.  Again, identifier for an
   alignment.  The SPF identifier to align
   in DMARC, the validated domain in RFC7208.MAILFROM must be part
   of the same Organizational Domain as the domain in the RFC5322.From
   header field.
   Even field to be aligned.  It is important to note that even when
   an SPF record exists for the domain in RFC5322.From, SPF will not
   authenticate it unless it is also the domain SPF checks.
   While aligning RFC5322.From and RFC5321.MailFrom is usually possible,
   it can be difficult to change the domain in the HELO/EHLO used for
   bounces to the domain in the RFC5322.From header field, especially
   when several mail streams share the same sending IP address. RFC7208.MAILFROM,
   furthermore, RFC7208.MAILFROM definition is different from
   RFC5321.MailFrom [RFC5321] definition.

2.2.  Message Forwarding

   Message forwarding is a generic concept encapsulating a variety of
   behaviors.  Section 3 describes forwarding behavior as it relates to
   the components of the Internet Mail Architecture.

   All of these behaviors involve mail email being retransmitted by a new
   SMTP server.  As discussed above, for SPF to cause a DMARC pass, the
   domain of the RFC5321.MailFrom or RFC5321.HELO/EHLO RFC7208.MAILFROM, must be aligned with that of the
   RFC5322.From header field. field:

   o  If the RFC5321.MailFrom is not empty and if the forwarder keeps
      the RFC5321.MailFrom, the SPF validation will fail altogether
      unless the forwarder is an authorized part of the originator's mail
      email sending infrastructure.  If  On another hand, if the forwarder
      uses its own domain in the
   RFC5321.MailFrom and/or RFC5321.HELO/EHLO, RFC5321.MailFrom, SPF passes but the
      alignment with the RFC5322.From header field fails.

   o  If the RFC5321.MailFrom is empty, the RFC5321.Helo of the
      forwarder will likely be in different organizational domain of the
      RFC5322.From.  SPF may pass but the alignment with the
      RFC5322.From header field fails.

   In either case, SPF cannot produce a DMARC pass, and DKIM will be
   required to get DMARC to pass.

2.3.  Message Modification

   Modification of email content invalidates most DKIM signatures.  For
   instance while signatures, and
   many message forwarding systems modify email content.  Mailing list
   processors are the most common example of such systems, but other
   forwarding systems also make modifications.  Although DKIM provides a
   length flag so that content can be appended (See Section 8.2 of RFC6376
   [RFC6376] for additional security considerations), in practice,
   particularly with MIME-encoded [RFC2045] messages, a mailing list
   processor will do more than append (See Section 5.3 of [RFC5598] for
   details).  Even forwarding systems
   make content modifications.  Furthermore, the use of the length flag is by no means universal. seldom found in
   emails in part because of its security challenges.

   DKIM has two canonicalizations: canonicalizations to use for headers and body
   separately: simple and relaxed.  The latter allows some modest in
   transit modifications that do not change the interpretation of the
   content of the email.  The relaxed canonicalization used to be is more computing
   intensive and may not have been preferred in the early deployment of DKIM.
   DKIM as this may have been more significant than today.

3.  Internet Mail Architecture, DMARC, and Indirect Email Flows

   This section describes components within the Internet Mail
   Architecture [RFC5598] where interoperability issues between DMARC
   and indirect email flows can be found.

3.1.  Message Handling System

   Section 4 of [RFC5598] describes six basic components that make up
   the Message Handling System (MHS):

   o  Message

   o  Message User Agent (MUA)

   o  Message Submission Agent (MSA)

   o  Message Transfer Agent (MTA)

   o  Message Delivery Agent (MDA)

   o  Message Store (MS)

   Of these components MSA, MTA, and MDA are discussed in relation to
   interoperability with DMARC.

   A Mediator is a special class of MUA that is given special
   consideration in this section due to the unique issues Mediators face
   when attempting to interoperate with DMARC.

3.1.1.  Message Submission Agents

   An MSA accepts messages submitted by a Message User Agent (MUA) and
   enforces the policies of the hosting ADministrative Management Domain
   (ADMD) and the requirements of Internet standards.

   MSAs are split into two sub-components:

   o  Author-focused MSA functions (aMSA)

   o  MHS-focused MSA functions (hMSA)

   MSA interoperability issues with DMARC begin when an aMSA accepts a
   message where the RFC5322.From header field contains a domain that is
   outside of the ADMD of the MSA.  The ADMD will almost certainly not
   be capable of sending email that yields Authenticated Identifiers
   aligned with the domain found in the RFC5322.From header field.
   Examples of this issue include "forward-to-friend" functionality
   commonly found on news/article websites or "send-as" functionality
   present on some MUAs.

   When an hMSA takes responsibility for transit of a message containing
   a domain in the RFC5322.From header field that is outside of the
   hMSA's ADMD, the hMSA faces DMARC interoperability issues if the
   domain publishes a DMARC policy of "quarantine" or "reject".  These
   issues are marked by an inherent difficulty in modifying establishing alignment
   with the domain present in a message's RFC5322.From header field.
   Examples of this issue include:

   o  Pseudo-open relays - a residential ISP that allows its customers
      to relay any domains through its infrastructure.

   o  Embedded devices - cable/dsl modems, firewalls, wireless access
      points that send email using hardcoded domains.

   o  Email service providers - ESPs that service customers that are
      using domains that publish a DMARC "reject" policy.

   o  Calendaring software - an invited member of an event modifies the
      event causing calendaring software to emit an update that appears
      to come from the creator of the event.

3.1.2.  Message Transfer Agents

   MTAs relay a message until the message reaches a destination MDA.  Message Encoding

   An MTA may change modify the message encoding, for instance by converting
   8-bit mail MIME sections to quoted-printable 7-bit sections.  This
   modification is outside the scope of DKIM canonicalization and will
   invalidate DKIM signatures that include message content.  Header Standardization

   An MTA may standardize headers, usually in order to make non-RFC
   compliant headers properly compliant.  For instance, some common MTAs
   will correct comprehensible but non-compliant date formats to
   compliant ones.  Again, this  This correction is outside the scope of DKIM
   canonicalization and will invalidate DKIM signatures.  This
   correction is also outside the scope of this document in providing
   solutions for non RFC compliant emails.  Email Address Internationalization

   A DMARC interoperability issue arises in the context of Email Address
   Internationalization [RFC6530].  [RFC6854] allows group syntax in the
   RFC5322.From header field during the transition period to SMTPUTF8.
   If an EAI/SMTPUTF8-aware MTA needs to transmit a message to a non-
   aware MTA, the EAI/SMTPUTF8-aware system may transform the
   RFC5322.From header field of the message to include group syntax to
   allow the non-aware MTA to receive the email.

   This transformation will modify the original content of the message
   and may invalidate any DKIM signatures if the transformation is not
   done by the MSA or MUA.  In addition, group syntax will remove the
   ability for the DMARC mechanism to find an Organizational Domain that
   aligns with any authenticated domain identifier from SPF or DKIM.

   In addition, the group syntax will may result in an invalid domain in the
   RFC5322.From header field. impossibility of
   finding a domain with a DMARC policy associated with it.  If the
   receiving MTA pays attention to the validity and reputation of
   domains, this may present its own set of delivery problems.  For
   instance an MTA may refuse emails with no valid or emailable domain
   in the RFC5322.From as to avoid simple workarounds against DMARC.

3.1.3.  Message Delivery Agents

   The MDA transfers a message from the MHS to a mailbox.  Like the MSA,
   the MDA consists of two sub-components:

   o  MHS-focused MDA functions (hMDA)

   o  Recipient-focused MDA functions (rMDA)

   Both the hMDA and the rMDA can redirect a message to an alternative
   address.  DMARC interoperability issues related to redirecting of
   messages are described in Section 3.2.

   SIEVE [RFC5228] functionality often lives in the rMDA sub-component
   and can cause DMARC interoperability issues.  The SIEVE 'addheader'
   and 'deleteheader' filtering actions can modify messages and
   invalidate DKIM signatures, removing DKIM-supplied Authenticated
   Identifiers as inputs to the DMARC mechanism.  There are also SIEVE
   extensions that modify the body.  SIEVE may only become an issue when
   the email is reintroduced in the transport infrastructure.

3.2.  Mediators

   See [RFC5598] for a complete definition of Mediators.

   Mediators forward messages through a re-posting process.  Mediators
   share some functionality with basic MTA relaying, but have greater
   flexibility in both addressing and content modifications.

   DMARC interoperability issues are prevalent within the context of
   Mediators, which are often used precisely for their ability to modify

3.2.1.  Alias

   An Alias is a simple re-addressing facility that provides one or more
   new Internet Mail addresses, rather than a single, internal one.  A
   message continues through the transfer service for delivery to one or
   more alternative addresses.

   Aliases can be implemented by mailbox-level forwarding (e.g. through
   "dot-forwarding") or SIEVE-level forwarding (through the SIEVE
   'redirect' action) or other methods.  When an Alias preserves message
   content and does not make significant header changes, DKIM signatures
   may remain valid.  However, Aliases often extend the delivery path
   beyond SPF's ability to grant authorization.

   Examples of Aliasing include:

   o  Forwarding email between freemail providers to try different
      interfaces while maintaining an original email address.

   o  Consolidating many email addresses into a single acccount to
      centralize processing.

   o  Services that provides "activity based", "role based" , "vanity"
      or "temporary" email addresses such as universities and
      professional associations.  For instance professional or alumni
      institutions may offer to their members an alias for the duration
      of their membership but may not want to deal with the long term
      storage of emails.

   In most cases, the aMSA providing Alias services has no
   administrative relationship to the ADMD of the final recipient, so
   solutions to Alias-related DMARC failure should not assume such a

3.2.2.  ReSenders

   ReSenders "splice" a message's addressing information to connect the
   Author of the original message with the Recipient Recipient(s) of the new
   message.  The new Recipient sees the message as being from the
   original Author, even if the Mediator adds commentary.

   ReSenders introduce DMARC interoperability issues as content
   modification invalidates DKIM signatures.  SPF's ability to grant
   authorization via alignment is removed as the new Recipient receives
   the message from the Mediator.

   Without an ability to produce Authenticated Identifiers relevant to
   the Author's RFC5322.From header field domain using either DKIM or
   SPF, the new Recipient has almost no chance of successfully applying
   the DMARC mechanism.

   Examples of ReSenders include MUA-level forwarding by resending a
   message to a new recipient or by forwarding a message "inline" to a
   new recipient (this does not include forwarding a message "as an
   attachment").  An additional example comes in the form of calendaring
   software that allows a meeting attendee (not the meeting organizer)
   to modify the content of an invite causing the invitations to appear
   to be reissued from the meeting organizer.

3.2.3.  Mailing Lists

   A Mailing List receives messages as an explicit addressee and then
   re-posts them to a list of subscribed members.  The Mailing List
   performs a task that can be viewed as an elaboration of the ReSender.

   Mailing Lists share the same DMARC interoperability issues as
   ReSenders (Section 3.2.2), and very commonly modify headers or
   message content in ways that will cause DKIM to fail, including:

   o  prepending the RFC5322.Subject header field with a tag, to allow
      the receiver to identify visually the mailing list.

   o  adding a footer to the email body to contain administrative

   o  removing some MIME-parts from the email or converting the message
      to text only.

   o  PGP-encrypting the body to the receiver's key.

   o  enforcing community standards by rewriting banned words.

   o  allowing moderators to add arbitrary commentary to messages.

   Any such modifications would invalidate a DKIM signature.

   Mailing Lists may also have the following DMARC interoperability

   o  Subscribed members may not receive email from members that post
      using domains that publish a DMARC "p=reject" policy.

   o  Mailing Lists may interpret DMARC-related email rejection as an
      inability to deliver email to the recipients that are checking and
      enforcing DMARC policy.  This processing may cause subscribed
      members to be suspended or removed from the Mailing List.
      [RFC3463] specifies Enhanced Mail System Status Codes which help
      differentiate between various bounces.  DMARC even defines
      specific codes to be used.

3.2.4.  Gateways

   A Gateway performs the

   These changes are common for many mailing lists and receivers are
   used to them.  Furthermore MUA expects certain mailing list behavior
   in presenting emails to the end users

3.2.4.  Gateways

   A Gateway performs the basic routing and transfer work of message
   relaying, but it also is permitted to modify content, structure,
   address, or attributes as needed to send the message into a messaging
   environment that operates under different standards or potentially
   incompatible policies.

   Gateways share the same DMARC interoperability issues as ReSenders
   (Section 3.2.2).

   Gateways may share also the same DMARC interoperability issues as
   MTAs (Section 3.1.2).

   Gateway-level forwarding can introduce DMARC interoperability issues
   if the Gateway is configured to rewrite the message to map between
   recipient domains.  For example, an acquisition may lead the
   acquiring company to decide to decommission the acquired companies company's
   domains by rewriting messages to use the domain of the acquiring
   company.  Since the To: RFC5322.To header field is usually DKIM-signed,
   this kind of rewriting will also cause DKIM signatures to fail.

3.2.5.  Boundary Filters

   To enforce security boundaries, organizations can subject messages to
   analysis for conformance with their safety policies.  A filter might
   alter the content to render it safe, such as by removing content
   deemed unacceptable.

   Boundary Filters share the same DMARC interoperability issues as

   Issues may arise if SPF and DKIM is evaluated after the filter

   Examples of Boundary Filters include:

   o  Anti-spam: To keep its reputation, an MTA that transfers a message
      may remove harmful content from messages that are likely to be
      unwanted by the next MTA and/or add text in the body to indicate
      the message has been scanned.  Any such modifications would
      invalidate a DKIM signature.

   o  Any service that reformulates the RFC5322.body for any other
      reason, for instance adding an organizational disclaimer.

   o  Secondary MX services.  In this case, however, it is inappropriate
      for a primary MX server to perform an SPF check against its own
      secondaries.  Rather, the secondary MX should perform this

3.3.  Combinations

   The causes of indirect email flows can be combined.  For example, a
   university student may subscribe to a mailing list (using his
   university email address) while this university email address is
   configured to forward all emails to a freemail provider where a more
   permanent email address for this student exists.

   Within an organization the message may pass through various MTAs
   (Section 3.1.2), each of which performs a different function
   (authentication, filtering, distribution, etc.)

4.  Possible Solutions to Mitigations of Interoperability Issues

   Solutions to interoperability issues between DMARC and indirect email
   flows vary widely in their scope and implications.  They range from
   improvements to underlying processors, such as proper handling of
   multiple DKIM signatures, to more radical approaches to the messaging
   architecture.  This section describes possible ways to address
   interoperability issues.

   Mail systems are diverse and widely deployed and are expected to
   continue to work with old systems.  For instance, Qmail is still used
   and the base code has not been updated since 1998.  Ezmlm, a once
   popular mailing list manager, is still deployed and has not been
   updated since 1997, although a new version, Ezmlm-idx exists.  In
   this constrained environment, some solutions may be time-consuming
   and/or disruptive to implement.

   DMARC provides for receivers to make decisions about identity
   alignment acceptability based on information outside the DMARC
   headers and
   communicate those decisions as "overrides" to the sender.  This
   facility can be used to ease some interoperability issues, although
   care is needed to ensure that this does not create loopholes that
   abusers can use arbitrarily.

4.1.  Mitigations in Current Use

   At many places where DMARC is already deployed, mitigations are in
   use.  These mitigations vary in their effectiveness and side effects,
   but have the advantage that they are currently available.

4.1.1.  Mitigations for Senders  Identifier Alignment

   Currently used work-arounds and fixes to identifier alignment issues:

   o  MTAs handling multiple domains may choose to change
      RFC5321.MailFrom to align with RFC5322.From to improve SPF
      usability for DMARC.

   o  MTAs handling multiple domains may also choose to align HELO/EHLO
      RFC5321.HELO/EHLO to RFC5322.From, particularly when sending
      bounce messages.  Adjusting dynamically the RFC5321.HELO based on
      the RFC5322.From may not be possible for some MTA software.

   o  MTAs may choose to DKIM sign bounces with an aligned domain to
      allow DKIM-based DMARC pass.

   o  MTAs handling multiple domains may require DMARC-using senders to
      provide DKIM keys and use DKIM to avoid SPF alignment issues.
      Handling DKIM keys with a third party has its security challenges.

   o  ReSenders  Senders who are sending on behalf of users in other Administrative
      Domains may choose to change use an RFC5322.From to one under the
      ReSender's control, avoiding alignment issues with sender's
      control.  The new From can be either a forwarding address in a
      domain controlled by the original.

   o  Receivers should update DKIM handling libraries to ensure that
      they process all valid DKIM signatures and check them for

   Proposed and in-progress work-arounds and fixes to identifier
   alignment issues:

   o  Third party authorization, [RFC6541], [I-D.otis-tpa-label] and
      [I-D.kucherawy-dkim-delegate] provide ways to extend identifier
      alignment under Sender, or a placeholder address, with
      the control of original user's address in a RFC5322.Reply-to header field.
      However this may affect what the domain owner.

4.2. recipient expects in its MUA.  Message Modification

   Message modification invalidates DKIM signatures and complicates a
   receiver's ability to generate Authenticated Identifiers from a
   message.  Avoiding message modification wherever possible is
   therefore desirable.

   Currently used work-arounds and fixes to message modification issues:

   o  Senders can maximize survivability of DKIM signatures by limiting
      the header fields they sign, using relaxed canonicalization and by
      using length to allow appended signatures.

   o  Senders can also maximize survivability by starting with RFC-
      compliant headers and common body formats.

   o  Forwarders can choose  In order to add email headers instead minimize the chances of modifying
      existing headers or bodies.

   o  Forwarders transport conversions, Senders
      can minimize convert the circumstances in which they choose message to
      fix messages, preferring preserving non-compliant headers a suitable MIME content-transfer
      encoding such as quoted-printable or base64 before signing
      ([RFC6376] Section 5.3).

4.1.2.  Mitigations for Receivers  Identifier Alignment

   o  Receivers should update DKIM handling libraries to
      creating ensure that
      they process all valid DKIM failures. signatures and check them for
      alignment.  Policy Override

   o  Forwarders  Receivers can choose amalgamate data from their user base to reject messages with suspect or harmful
      content instead of modifying them.

   o  If message modification is required, the RFC5322.From identify
      forwarders and use such list for a DMARC local policy override.
      This process may be

   Proposed and in-progress work-arounds easier for large receivers, where there is
      data and fixes resources to message
   modification issues: create such lists, than for small receivers.  Email Address Internationalization

   o  DKIM with constrained transformations,

4.3.  Message Forwarding

   Forwarding messages without modification is referred  During the transition from email systems that do not allow EAI
      (SMTPUTF8) to as
   "transparent forwarding", and email systems that allow it, [RFC6854] allows the
      MUA or MSA to use the group syntax for the RFC5322.From header
      field to avoid a known MTA to reject the message (if RFC5322 is
      implemented strictly).  While this will alleviate the EAI problem,
      it will allow a way simple DMARC workaround since the message may no
      longer have a single usable domain in the RFC5322.From header
      field, making DMARC inapplicable.  DMARC implementations that pay
      attention to preserve the validity of
   DKIM signatures.

   Currently used work-arounds and fixes the domains in the RFC5322.From may
      then have to message forwarding issues:

   o  Senders should use DKIM signing choose between EAI transitioning compliance
      (accepting the email without a domain in RFC5322.From) and
      security (rejecting RFC-compliant email for lack of a domain in
      the RFC5322.From).  The real solution is to allow transparent forwarding upgrade MTAs to

      support EAI (SMTPUTF8), avoiding the transition issue.

4.1.3.  Mitigations for ReSenders may choose  Changes to change the RFC5322.From

   Many ReSender issues can be avoided by using an RFC5322.From to one under
   the ReSender's control, avoiding instead of the initial RFC5322.From.  This
   will correct identifier alignment issues with and allow arbitrary message
   modification, for instance.  When ReSenders change the RFC5322.From,
   it is desirable to preserve the information about the original
   initiator of the original. message.  The Original-From [RFC5703] (or X-Original-From) X-
   Original-From) header field is used for this purpose in various
   contexts (X- header fields name are discouraged by [RFC6648]).

   Note that

   However, handling of Original-From (or X-Original-From) is merely adding
   complexity to the 'who was the author of this message' assessment,
   very possibly creating yet-another security hole.

4.3.1.  Original-Authentication-Results

   [I-D.kucherawy-original-authres] has been mentioned in early DMARC
   drafts as a way to pass along Original Authentication Results to
   "downstream" receivers.

4.4.  Message Handling Services

4.4.1.  Message Transfer Agents  Encoding

   There are few reasons to modify the encoding of the message,
   compatibility issues between international character sets are few
   nowadays.  More mail systems supports 8bitMIME, therefore the need
   for transport encoding changes are rarer.  By default no modification
   of the message should be done when simply forwarding the message.  Filters

   Filters should not add to or modify the body of the message, but
   either should reject the message or add new email headers (not under
   DKIM) to indicate the result of the filter.  Email Address Internationalization

   During the transition from email systems that do X-Original-From) is not allow EAI
   defined anywhere.  It is not currently used consistently or displayed
   to email system that allows it, [RFC6854] allows using the
   group syntax for the RFC5322.From header field rather than rejecting the message (if RFC5322 user, but in any situation where it is implemented strictly).  Allowing the group
   syntax used, it is at the appreciation of the postmaster, that will always a new
   unauthenticated identifier available for exploitation.  Avoiding Message Modification

   o  Forwarders can choose the solution best to add email header fields instead of
      modifying existing headers or bodies, for its user, but really instance to avoid DMARC not
   finding indicate a single useable domain in the RFC5322.From header field,
      message may be spam.

   o  Forwarders can minimize the
   real solution is circumstances in which they choose to upgrade your MTAs,
      fix messages, preferring to support EAI (SMTPUTF8).  In
   that case a sending SMTPUTF8 MTA does not need preserve non-compliant headers to require a downgrade
   of the message
      creating DKIM failures.

   o  Forwarders can choose to ASCII identifiers.  Encouraging, by rejection reject messages with suspect or
   reputation scoring, the presence harmful
      content instead of a domain in the RFC5322.From
   header field is easier.

4.5.  Mediators

4.5.1. modifying them.  Mailing Lists

   [RFC6377] provides some guidance on using DKIM with Mailing lists.
   Here are some other remediations remediation techniques:

   o  One common mitigation policy is to configure the Mailing List
      Manager (MLM) to alter the RFC5322.From header field to use the
      domain of the MLM.  Since most list subscribers prefer to know the
      identity of the author of the original message, typically this
      information may be provided in the display name part of the
      RFC5322.From header field.  This display name needs to be
      carefully crafted as to not collide with the original display name
      of the author, nor contain something that looks like an email
      address or domain name.  These modification modifications may to some extent
      defeat the purpose of DMARC itself.  It may make it difficult to
      ensure that users of all email clients can easily reply to the
      author, the list, or all using the email client features provided
      for that purpose.  Use of "Reply-To" RFC5322.Reply-To header field can
      alleviate this problem depending
      if on whether the mailing list is
      configured to reply-to-list, reply-to-
      author reply-to-author or reply-to-fixed-
      address, however it is important to note that this header field
      can take multiple email addresses.  When altering the RFC5322.From
      there are two possibilities, to change it to put the mailing list
      email address, or reply-to-fixed-address. to change it to add a suffix like ".invalid" to
      the domain of the email address present there.  The later
      modification may create issues because it is an invalid domain
      name, and some MTAs may take particular attention to the validity
      of email addresses in RFC5322.From and the reputation of the
      domains present there.

   o  Another common mitigation policy is to configure the MLM to "wrap"
      the message in a MIME message/rfc822 part.  This completely
      bypasses the DMARC policy in clients that allow reading MLM to "wrap"
      the message in a MIME message/rfc822 part and to send as a message. the
      Mailing List email address.  Many email clients (as of August
      2014) have difficulty reading such messages.

   o  Finally a  A, now, less common mitigation policy, is to configure the MLM to
      not modify the message so that the DKIM signature remains valid.

   o  To alleviate unsubscribes to the mailing list due to the messages
      bouncing because of DMARC, the MLM needs to not act on bounces due
      to Message Authentication issues.  [RFC3463] specifies Enhanced
      Mail System Status Codes which help differentiate between various
      bounces.  Correctly interpreting Extended SMTP error messages is
      useful in this case ([RFC7372]). in particular codes defined in [RFC7372] and
      in DMARC.

   All these techniques may provide some specific challenges in MUAs and
   different operational usages for end users (like rewriting filters to
   sort emails in folders).  There will be some time before all
   implications are understood and alleviated.


4.2.  Proposed and In-Progress Mitigations

   The following mitigations are based on Internet Drafts which have not
   yet received broad consensus.  They are described here to offer
   exploratory path for solutions.  These solutions should not be used
   in a production environment.

   o  Third party authorization, [RFC6541], [I-D.otis-tpa-label] and
      [I-D.kucherawy-dkim-delegate] provide ways to extend identifier
      alignment under the control of the domain owner.

   o  DKIM with constrained transformations,
      [I-D.kucherawy-dkim-list-canon] is proposed to allow message

   o  DKIM with recorded transformations, [I-D.kucherawy-dkim-transform]
      is proposed to indicate what limited transformations were done to
      the message so that a receiver could reverse them and confirm the
      validity of the orignal DKIM signature.

   o  [I-D.kucherawy-original-authres] is intended as a way to pass
      along Original Authentication Results to "downstream" receivers.

      It is not widely implemented and relies on a trust relationship
      between the forwarder and the other receivers.

   o  [I-D.levine-dkim-conditional] could be used to have the sender add
      a limited DKIM signature, that signs only a very limited set of
      header fields and not the body of the message.  This DKIM
      signature would come with the condition that a subsequent known
      domain fully DKIM sign the message.  For instance a Mailing List
      could transform the message, add its DKIM signature and there
      would be a valid DKIM signature aligned with the RFC5322.From that
      would satisfy DMARC while limiting the possibilities of replay

4.2.1.  Getting More Radical: Requiring New Communication Paths Between
      MUA and the Message Store

   In practice a number of operators are using strict alignement alignment mode in
   DMARC in order to avoid receiving new and innovative forms of
   unwanted and unauthentic mail email through systems purporting to be
   mailing list handlers.  The receiving ADMD has no knowledge of which
   lists the user has subscribed to and which they have not.  One avenue
   of exploration would be for the user to authorize mailing lists as
   proxies for authentication, at which point the receiving ADMD would
   be vesting some trust in the mailing list service.  The creators of
   DKIM foresaw precisely this possibility at the time by not tightly
   binding any semantics to the RFC5322.From header field.  Some
   experimental work has taken place in this area, as mentioned above.
   Additional work might examine a new communication path to the user to
   authorize third party signatures. some form of transitive trust.

5.  IANA Considerations

   This document contains no actions for IANA.  [RFC Editor: Please
   delete this section prior to publication.]

6.  Security Considerations

   This document is an analysis of DMARC's impact on indirect email
   flows.  It describes the possibility of accidental denial-of-service
   that can be created by rejections of messages by DMARC-aware Mail
   Receivers.  However, it introduces no new security issues to Internet

7.  Acknowledgments

   Miles Fidelman, John Levine, David Crocker, Stephen J.  Turnbull,
   Rolf E.  Sonneveld, Tim Dragen Draegen and Franck Martin contributed to the
   IETF DMARC Working Group's wiki page listing all known
   interoperability issues with DMARC and indirect mail email flows.

   Tim Draegen created the first draft of this document from these
   contributions and by carefully hamfistedly mapping contributions into the
   language of [RFC5598].

8.  References

8.1.  Normative References

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

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
              3463, January 2003.

   [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering
              Language", RFC 5228, January 2008.

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

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

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598, July

   [RFC5703]  Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part
              Tests, Iteration, Extraction, Replacement, and Enclosure",
              RFC 5703, October 2009.

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

   [RFC6377]  Kucherawy, M., "DomainKeys Identified Mail (DKIM) and
              Mailing Lists", BCP 167, RFC 6377, September 2011.

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

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

   [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648, June 2012.

   [RFC6854]  Leiba, B., "Update to Internet Message Format to Allow
              Group Syntax in the "From:" and "Sender:" Header Fields",
              RFC 6854, March 2013.

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

   [RFC7372]  Kucherawy, M., "Email Authentication Status Codes", RFC
              7372, September 2014.

8.2.  Informative References

              Kucherawy, M. and D. Crocker, "Delegating DKIM Signing
              Authority", draft-kucherawy-dkim-delegate-01 (work in
              progress), June 2014.

              Kucherawy, M., "A List-safe Canonicalization for
              DomainKeys Identified Mail (DKIM)", draft-kucherawy-dkim-
              list-canon-00 (work in progress), June 2014.

              Kucherawy, M., "Recognized Transformations of Messages
              Bearing DomainKeys Identified Mail (DKIM) Signatures",
              draft-kucherawy-dkim-transform-00 (work in progress),
              April 2015.

              Chew, M. and M. Kucherawy, "Original-Authentication-
              Results Header Field", draft-kucherawy-original-authres-00
              (work in progress), February 2012.

              Levine, J., "DKIM Conditional Signatures", draft-levine-
              dkim-conditional-00 (work in progress), June 2014.

              Otis, D. and D. Black, "Third-Party Authorization Label",
              draft-otis-tpa-label-00 (work in progress), May 2014.

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

Authors' Addresses

   Franck Martin (editor)
   Mountain View, CA


   Eliot Lear (editor)
   Cisco Systems GmbH
   Richtistrasse 7
   Wallisellen, ZH  CH-8304

   Phone: +41 44 878 9200

   Tim Draegen (editor)
   Eudaemonic Development LLC
   PO Box 19443
   Asheville, NC  28815


   Elizabeth Zwicky (editor)
   Sunnyvale, CA