What is "dmarc"

Domain-based Message Authentication, Reporting and Conformance (DMARC) is an email-validation system designed to detect and prevent email spoofing. It provides a mechanism which allows a receiving organization to check that incoming mail from a domain is authorized by that domain's administrators and that the email (including attachments) has not been modified during transport. It is thus intended to combat certain techniques often used in phishing and email spam, such as emails with forged sender addresses that appear to originate from legitimate organizations. DMARC is specified in RFC 7489.

DMARC is built on top of two existing mechanisms, Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM). It allows the administrative owner of a domain to publish a policy on which mechanism (DKIM, SPF or both) is employed when sending email from that domain and how the receiver should deal with failures. Additionally, it provides a reporting mechanism of actions performed under those policies. It thus coordinates the results of DKIM and SPF and specifies under which circumstances the From: header field, which is often visible to end users, should be considered legitimate.

DMARC's validation of the From field has similarities to {Author Domain Signing Practices} (ADSP, originally called DKIM Sender Signing Practices, DKIM-SSP). The reporting aspect builds on Abuse Reporting Format (ARF).