Essentially, Google Mail's SMTP server tells you that either the PTR record with IPv6 address of your SMTP server does not exist or this record does not match your server.
- Look at the DNS configuration of your SMTP server.
- If it does not have a PTR record with the IPv6 address of that server, add it.
- If your configuration does not allow adding PTR records, disable sending over IPv6 in your SMTP configuration. For Postfix, ensure that the parameter
inet_protocols(for example, in the file
/etc/postfix/main.cf) looks like
inet_protocols = ipv4, i.e. without