How email works
The standards documents behind everything the tools on this site check, with plain-English notes on what each one covers.
SMTP & message format
How mail moves and what a message looks like.
MIME
Attachments, character sets and everything beyond plain ASCII.
Authentication
Proving who sent a message.
RFC 7208Sender Policy Framework (SPF)Which servers may send mail for a domain. Obsoletes RFC 4408.↗RFC 6376DomainKeys Identified Mail (DKIM)Cryptographic signatures tying mail to a domain.↗RFC 8463Ed25519 for DKIMModern short keys for DKIM signatures.↗RFC 7489DMARCPolicy and reporting on top of SPF and DKIM.↗RFC 8601Authentication-Results HeaderHow receivers record SPF/DKIM/DMARC verdicts.↗RFC 8617Authenticated Received Chain (ARC)Preserving authentication through forwarders and lists.↗
List management
Unsubscribing and bulk-sender requirements.
Feedback loops & reporting
How receivers report spam complaints back to senders.
Transport security
Encrypting mail in transit.