Resources
What is DMARC?
DMARC tells receivers what to do when SPF and DKIM disagree with the visible sender, and where to send reports.
DMARC requires alignment: the domain the recipient sees must match the domain that passed SPF or DKIM. Passing a check for an unrelated domain is not enough.
The policy can be none, quarantine or reject. Start at none with reporting, read what comes back, and only then tighten.
The rua address receives aggregate reports. It has to be a mailbox somebody reads — a DMARC record pointing at a dead address tells you nothing.
Reports are how you find the senders you forgot: the newsletter tool, the ticket system, the old server.