DMARC
Definition
Domain-based Message Authentication, Reporting, and Conformance (DMARC) is an email authentication protocol designed to give domain owners the ability to protect their domain from unauthorized use, such as email spoofing, by providing instructions on how to handle unauthenticated emails.
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Translation
The bouncer for your email domain—DMARC checks who's trying to send emails in your name and either lets them in, kicks them out, or keeps an eye on them, making sure no one’s impersonating you and causing trouble.