This article explains what DMARC is and where to find the documentation to set up a policy for your company.

The implementation of a DMARC (Domain-based Message Authentication, Reporting and Conformance) policy allows a sender to indicate that his emails are protected by DKIM and/or SPF and tells receiving servers what should be done when receiving an unauthenticated email - for example, reject the email or report it as spam.

DMARC eliminates uncertainty at reception: although the SPF protocol and DKIM signature help to prove that an email is legitimate, their absence or failure does not prove the opposite.

Your DMARC policy addresses this uncertainty by allowing you to explicitly indicate what should be done with an unauthenticated email.

 PrerequisiteTo implement a secure DMARC policy that rejects emails that fail authentication, it is necessary that all emails are authenticated correctly (see About Email Authentication).

How to

The site https://dmarc.org contains all the resources necessary to enable your team to understand DMARC, prepare an appropriate policy, and test it.

Some more technical tools are also listed in this section: https://dmarc.org/resources/deployment-tools/