http://eco.senritu.net/gmail%E3%81%AB%E3%83%A1%E3%83%BC%E3%83%AB%E3%83%AA%E3%83%AC%E3%83%BC%E3%81%99%E3%82%8Bpostfix%E3%83%A1%E3%83%BC%E3%83%AB%E3%82%B5%E3%83%BC%E3%83%90/

https://tech.matchy.net/archives/35

http://park1.wakwak.com/~ima/centos_postfix_relay0001.html