TL;DR:

Imagine you run a business and want to ensure that all the letters you send are trusted and not tampered with. DMARC (Domain-based Message Authentication, Reporting, and Conformance) is like a set of instructions you give to the post office. These instructions tell the post office how to handle your letters if they suspect they are fake. If the letter is genuine, it gets delivered. If not, it might be held for further inspection or returned to the sender. This helps prevent scammers from sending fake letters pretending to be from your business.

Details For the Techies:

DMARC (Domain-based Message Authentication, Reporting, and Conformance) is an email authentication protocol designed to protect email domains from unauthorized use, such as email spoofing. It builds on two existing mechanisms, SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail), to provide a way for domain owners to publish policies in their DNS records. These policies specify how to handle emails that fail authentication checks. When an email is received, the DMARC policy instructs the receiving server on what to do if the email fails SPF or DKIM checks—whether to deliver, quarantine, or reject the email. DMARC also provides a reporting mechanism, allowing domain owners to receive feedback on email authentication results. This protocol is crucial for preventing phishing, business email compromise, and other cyber threats by ensuring that only legitimate emails are delivered.

For more detailed information, you can refer to the Wikipedia article on DMARC.