https://docs.google.com/document/d/1zuh7JJ053IbvNesxUavtfGYB_kkD9KbZ9YEMuk2ZM9Q ← Email History Timeline (not comprehensive)
https://www.ftc.gov/business-guidance/resources/can-spam-act-compliance-guide-business ← CAN SPAM Act of 2003
SPF
https://dmarcian.com/spf-syntax-table/ ←SPF Syntax table
https://easydmarc.com/tools/spf-lookup ←Check SPF record, give you the whole tree
https://mxtoolbox.com/spf.aspx ←Look up the record, test specific IP, get some health feedback
https://dmarcian.com/spf-survey/ ← Good summary, also shows the tree
DKIM
https://knowledge.validity.com/s/articles/DKIM-DNS-record-overview ← Comprehensive list of DKIM record options
https://support.google.com/a/answer/180504?hl=en ← Enable DKIM in Google Workspace
https://learn.microsoft.com/en-us/defender-office-365/email-authentication-dkim-configure ← Enable DKIM in MS 365
https://www.cloudflare.com/learning/dns/dns-records/dns-dkim-record/ ←Good information about DKIM
DMARC
https://mxtoolbox.com/dmarc/details/what-is-a-dmarc-record ←DMARC Description
https://dmarcian.com/dmarc-inspector/ ← Look up your record, gives some feedback on how good it is.
https://mxtoolbox.com/dmarc.aspx ← Record lookup, some indication of health.
https://mxtoolbox.com/dmarc/details/dmarc-tags ← DMARC Tag list, with links to details on each of them
Reporting Tools:
https://dmarc.postmarkapp.com/ ← Free, but limited reporting, good for an overview.
https://dmarcdigests.com/ ← $14/mo per domain
Test email deliverability based on all three: https://www.dmarctester.com/