The short answer is yes, DKIM or a domain key is a critical part of the authentication process. The reason is that DKIM (domain key) is one of the crtical elements of email authentication
is because the DKIM public/private Keys contain vital information for receiving mail servers to verify that the MAIL:FROM has been signed by the mail server with
a private key that matches the public key published in the DNS.
The reason why this was not a sufficient authentication process on its own is because email often has two email addresses (see how email works ) being used to send the email. The MAIL:FROM and the SMTP:FROM. The SMTP:FROM
is authenticated used an SPF record.