What is

DNS and email. Short answers, then the document.

• What is DMARCDMARC is a TXT record at _dmarc plus the domain. It tells the receiver what to do when the From address is not aligned with a passing SPF or DKIM result, and where to send the reports.
• What is SPFSPF is a TXT record that starts with v=spf1. It lists the hosts allowed to send mail for the domain. Two records that both start with v=spf1 on the same name are a permerror.
• What is DKIMDKIM signs a message. The public key is a TXT record under _domainkey. The signature's s= tag is the selector and d= is the signing domain.
• What is DNSSECDNSSEC adds data origin authentication and data integrity to DNS. A resolver can check that the answer was signed by the zone. The extensions do not provide confidentiality.
• What is MTA-STSMTA-STS publishes a policy that tells sending mail servers to use TLS, and which hosts may receive the mail. The policy id is a TXT record at _mta-sts. The policy itself is HTTPS.