参考文献
• Domain-based Message Authentication, Reporting, and
Conformance (DMARC)
https://tools.ietf.org/html/rfc7489
• DNS security | Cloudflare UK
https://www.cloudflare.com/en-gb/learning/dns/dns-security/
• Sender Policy Framework (SPF) for Authorizing Use of
Domains in Email, Version 1
https://tools.ietf.org/html/rfc7208