DMARC is an email authentication protocol that builds on SPF and DKIM. It allows domain owners to:
Monitor Policy: P=none - The none policy only gives insight in who’s sending email on behalf of a domain and will not affect the deliverability.
Quarantine policy: P=quarantine - The quarantine policy will already mitigate the impact of spoofing, but spoof emails will still be delivered to the receiver (in the spam folder)
Reject Policy: P=reject - This policy makes sure that all incorrect setup emails (spoofing emails) will be deleted by the email receiver and not land in the inbox of the receiver.