Contents
sending hostname の条件は成立しそうもない。
1. IP addresses
Your sending IP address must have a PTR record. PTR records verify that the sending hostname is associated with the sending IP address.
Every IP address must map to a hostname in the PTR record. The hostname specified in the PTR record must have a forward DNS that refers to the sending IP address.
Set up valid reverse DNS records of your sending server IP addresses that point to your domain. Check for a PTR record with the Google Admin Toolbox Dig tool.
Important: The sending IP address must match the IP address of the hostname specified in the Pointer (PTR) record.