使用 AWS SES 寄出通知信時，尤其是 gmail 時，會顯示為 via amazonses.com，然後此封 email 會被歸類為 spam
需要設定 DKIM 驗證，來避免此狀況
Reference
- How to prevent from gmail showing via amazonses.com? - MailBluster
- How to Set Up AWS SES and Avoid Spam Folders | by Rob Wilson | AWS in Plain English
- What is a DNS DKIM record? | Cloudflare
- 設定 SPF, DKIM, DMARC Email 驗證解決「Gmail 未經驗證網域」警告 - Soul & Shell Blog
- What is a DNS SPF record? | Cloudflare