[Reference:, https://www.cisco.com/c/en/us/products/security/what-is-dmarc.html, , SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) are two email authentication mechanisms that are utilized during this process. SPF and DKIM allow the domain owner to publish DNS records that specify the authorized IP addresses or hosts for sending emails from that domain and sign the messages with a cryptographic key to prove their authenticity and integrity., DMARC (Domain-based Message Authentication, Reporting and Conformance) is an email authentication standard that builds on SPF and DKIM and allows the domain owner to publish DNS records that specify how receivers should handle messages that fail SPF or DKIM verification, such as reject, quarantine, or none, and how to report back the results of DMARC validation., The other options are not valid email authentication mechanisms that are utilized during this process, because they are not part of DMARC standard., References: [User Guide for AsyncOS 15.0 for Cisco Secure Email Gateway], page 11-2 and page 11-3., , , ]