https://single9.net/2016/01/ubuntu-%e8%ae%93-sendmail-%e4%bb%a5-gmail-%e7%9a%84-smtp-%e4%be%86%e7%99%bc%e9%80%81%e4%bf%a1%e4%bb%b6/
Ubuntu - 讓 Sendmail 以 Gmail 的 SMTP 來發送信件