The implementation of a DMARC policy (Domain-based Message Authentication, Reporting and Conformance) 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.
Did you know having a DMARC policy is required if you send more than 5000 emails daily?
The good news is that Dialog Insight takes care of it for you if you choose domain delegation when configuring your account. If you decide to configure your domains by yourself, you will have to set up a DMARC policy. For more information, see https://dmarc.org/resources/deployment-tools/.