It's pretty verbose and lengthy, but I recently read the NIST "Trustworthy Email" publication and it did a great job explaining these technologies - Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain Message Authentication, Reporting, and Conformance (DMARC) - that are used for modern email authentication.

https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.S... (PDF obv.)