DMARC, which stands for Domain-based Message Authentication, Reporting & Conformance, is an email authentication protocol. It is based on widely implemented SPF and DKIM protocols, it also adds a reporting feature that allows senders and recipients to improve and monitor domain protection from fraudulent email.
Spammers often "doctor" or "spoof" the "sender addresses" on emails, and make it look like it came from their domain. To prevent such abuse using your domain and to let other recipient domains know about your outgoing domain policies, you can publish a DMARC record using email services that use DMARC standards capable of handling unauthenticated mail. This also helps in controlling "spoofing" activities through your domain and helps to protect your domain reputation.
Before publishing DMARC
A DMARC policy allows the sender to indicate that their emails are SPF or DKIM protected and instructs recipients on the action if both SPF and DKIM checks fail, such as quarantining or rejecting a message. DMARC helps the receiver to better handle failed messages better and thereby limits or removes the exposure of end recipients to such spoofed emails using the domain. DMARC also provides a way for the email receiver to inform the sender about emails that pass or fail the DMARC assessment.
The DMARC policy will be effective only if you send all email using your own domains. Emails sent on behalf of your domain via third party services will appear unauthenticated and may be rejected according to the published DMARC policy. To authorise emails via third party providers, you must either share the DKIM key for inclusion in the headers, or emails must be sent via SMTP servers that have already published authorised SPF records and DKIM keys.
You must configure the SPF records and DKIM keys for your domains before publishing the DMARC policy. The DMARC policy relies on SPF and DKIM keys to ensure email authenticity. An email using your domains email address, which fails the SPF test and/or the DKIM test, will trigger the DMARC policy.