DMARC
Policy layer telling receivers what to do when SPF/DKIM fail, plus reporting.
Definition
DMARC builds on SPF and DKIM. It requires identifier alignment with the visible from-domain and lets you publish a policy (none, quarantine, reject) while receiving aggregate reports about who sends as you.
Why it matters
DMARC reduces spoofing and is part of bulk-sender expectations at major mailbox providers.