Internet email is defined across many standards rather than one document. Useful anchors include RFC 5321 for SMTP, RFC 5322 for Internet message format, RFC 6376 for DKIM, RFC 7208 for SPF and RFC 7489 for DMARC’s original standards-track predecessor publication.
Standards evolve and may be updated or obsoleted. When exact normative behavior matters, verify the current RFC and errata rather than relying only on implementation folklore.