2020 dkim domain alignment for dmarc

How DKIM domain alignment affects DMARC authentication in 2020.

DMARC (Domain-based Message Authentication, Reporting and Conformance),
is an email authentication standard, developed to combat spoofed domain mail.

In the chapter “3.1. Identifier Alignment” it says:

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

   DMARC authenticates use of the RFC5322.From domain by requiring that
   it match (be aligned with) an Authenticated Identifier.
   
   -- https://tools.ietf.org/html/rfc7489#section-3.1

It simply means:

   when a sender authenticates their email using SPF and/or DKIM,  
   at least one of the domains must align with the sending From domain

It was not clear to us if a message could fail SPF or DKIM check
and still pass the DMARC authentication.

We tested it using a tool available to everyone: a Gmail mailbox.
To see the outcome, open the message and select “Show original”:

Test 1 - forwarded message: spf-fail, dkim-pass (aligned)
spf-fail dmarc-pass

Test 2 - broken dkim key: dkim-fail, spf-pass (aligned)
dkim-fail dmarc-pass

The result is evident, the message passes DMARC authentication if it occurs:
SPF and domain alignment <OR> DKIM and domain alignment

To pass the DMARC check, in some cases it is therefore important to validate the DKIM signature:
the signing domain (d=example.com) must be aligned with the From domain.

Examples of “DMARC-PASS” results that otherwise would not have worked:

Case 1 - forwarding breaks the SPF authentication

Result: DKIM alignment allows the message to pass the DMARC check.

Case 2 - the SPF domain provided by the ESP (Email Service Provider)
CANNOT be aligned with the From domain

Result: DKIM alignment allows the message to pass the DMARC check.