起因於一台 ap-northeast-1 的機器 (東京) 會使用 us-west-2 的 SES (美西，奧勒岡)，然後發現信延遲的有點嚴重，看 mail log 發現是因為連線 timeout。
查了以後發現在「Amazon SES SMTP Issues」這邊就有提到 EC2 instance 對 port 25 有限制：
You are sending to Amazon SES from an Amazon EC2 instance via port 25 and you cannot reach your Amazon SES sending limits or you are receiving time outs—Amazon EC2 imposes default sending limits on email sent via port 25 and throttles outbound connections if you attempt to exceed those limits. To remove these limits, submit an Amazon EC2 Request to Remove Email Sending Limitations. You can also connect to Amazon SES via port 465 or port 587, neither of which is throttled.
按照建議，直接走 port 587 就可以解決，另外一個方法是開 support ticket 請 AWS 解除：「How do I remove the throttle on port 25 from my EC2 instance?」。
Amazon EC2 throttles traffic on port 25 of all EC2 instances by default, but you can request that this throttle be removed for your instance at Request to Remove Email Sending Limitations (you must sign in with your root account credentials). Provide a description of your use case for sending email, and then choose Submit.
還是改走 port 587 比較簡單一點...