Email authentication & infrastructure guides

What Is SPF? Understand Sender Policy Framework, mechanisms and SPF authentication. SPF Too Many DNS Lookups Learn why SPF exceeds its DNS lookup limit and how to troubleshoot it. What Is DKIM? Understand DKIM signatures, DNS public keys and message verification. DKIM Selector Guide Find DKIM selectors and understand selector-based DNS lookups. What Is DMARC? Understand DMARC authentication, alignment, policy and reporting. DMARC none vs quarantine vs reject Compare the three common DMARC policy modes. Gmail & Yahoo Sender Requirements Review authentication, DNS, TLS and unsubscribe readiness concepts. Reverse DNS & PTR for Email Understand PTR records, reverse DNS and FCrDNS. SMTP STARTTLS Explained Understand STARTTLS, TLS handshakes, certificates and ciphers. MTA-STS Setup Guide Learn how DNS and HTTPS policy files protect mail transport. BIMI Explained Understand BIMI DNS records, logos, DMARC and provider behavior. How to Analyze Email Headers Understand Received headers, authentication results, ARC and routing.