How to sign emails to setup DKIM and DMARC
Please check out the following two modules to have your Drupal mails send by a mailserver. Apart from that it may already be sufficient to just set a SPF record for your domain as DKIM and DMARC has actually nothing to do with Drupal.



PHPMailer

https://www.drupal.org/project/phpmailer


  Adds SMTP support for sending e-mails using the PHPMailer library.
  Comes with detailed configuration instructions for how to use Google
  Mail as mail server.


SMTP Authentication Support

https://www.drupal.org/project/smtp


  This module allows Drupal to bypass the PHP mail() function and send
  email directly to an SMTP server. The module supports SMTP
  authentication and can even connect to servers using SSL if supported
  by PHP.




How To use an SPF Record to Prevent Spoofing &amp; Improve E-mail Reliability

https://www.digitalocean.com/community/tutorials/how-to-use-an-spf-record-to-prevent-spoofing-improve-e-mail-reliability


  A carefully tailored SPF record will reduce the likelihood of your
  domain name getting fraudulently spoofed and keep your messages from
  getting flagged as spam before they reach your recipients.