Free tools
Looks up the TXT record at "_dmarc.yourdomain.com" and extracts the declared policy (none, quarantine or reject), which defines what receivers should do with mail that fails SPF/DKIM.
"none" only monitors without blocking anything; "quarantine" sends suspicious mail to spam; "reject" rejects it outright. Without a DMARC record, there is no instruction for the receiver at all.
A weak or missing policy makes it easier for third parties to send mail spoofing your domain toward your own customers or vendors.
Frequently asked questions
The recommended path is to move gradually from "none" to "quarantine" and then "reject" once you confirm legitimate mail passes SPF/DKIM.
No, it complements them: DMARC defines what to do when SPF or DKIM fail, but needs at least one of them configured.