Sender Policy Framework:
A DNS record that specifies which mail servers are authorized to send email on behalf of your domain, preventing unauthorized servers from spoofing your domain.

DomainKeys Identified Mail:
A cryptographic signature added to email headers that proves the message hasn't been tampered with and genuinely came from the claimed domain.

Domain-based Message Authentication, Reporting and Conformance:
A policy that tells receiving email servers what to do with emails that fail SPF or DKIM checks, while providing reports on authentication results.

Mail Transfer Agent Strict Transport Security:
A policy that enforces encrypted (TLS) connections when other email servers send mail to your domain, preventing downgrade attacks and ensuring secure email delivery.