https://www.dragapp.com/blog/gmail-guide/
1. Google
Sender Guidelines の変更には言及がない。
Gmail training and help https://support.google.com/a/users/answer/9259748
Manage unwanted messages in Gmail https://support.google.com/a/users/answer/10864105?hl=en&ref_topic=9348490&sjid=7447259629385299605-AP
2. whois