Pro move: type dmarc:yourdomain.com or blacklist:1.2.3.4 straight into the box — it switches tools for you.
About this tool. DKIM puts a tamper-proof cryptographic signature on outgoing mail, published under a 'selector' in your DNS. Enter selector:yourdomain.com to check a specific selector, or just a domain and we'll probe two dozen common selectors used by the big providers.