DKIM References
Here are a few additional references that can help you understand the process of enabling DKIM signing for your domains.
Google G Suite Administrator Help, “About DKIM:”
https://support.google.com/a/answer/174124?hl=en
Microsoft Office365 Help, “Use DKIM to validate outbound email sent from your custom domain in Office365:”
https://technet.microsoft.com/en-us/library/mt695945
OpenDKIM:
http://opendkim.org/
Wikipedia entry for DKIM:
https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail
RFC 6376, “DomainKeys Identified Mail (DKIM) Signatures”
https://tools.ietf.org/html/rfc6376
Word to Wise blog, “A DKIM Primer Resurrected:”
https://wordtothewise.com/2016/04/a-dkim-primer-resurrected/