https://support.google.com/a/answer/3726730?hl=ja
https://sendgrid.kke.co.jp/blog/?p=8210
vi /etc/postfix/main.cf
inet_protocols = all
↓
inet_protocols = ipv4
http://pentan.info/server/linux/gmail_postfix_ipv6.html
cat /var/log/maillog | grep "status=sent"
http://www.blacklistalert.org/
http://www.senderbase.org
http://www.spamhaus.org/
https://www.invaluement.com/removal/