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.

The solution

1. Look at the DNS configuration of your SMTP server.
2. If it does not have a PTR record with the IPv6 address of that server, add it.
3. 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 ipv6.

Sources

• https://support.plesk.com/hc/en-us/articles/213936285-Unable-to-send-an-email-to-Gmail-Our-system-has-detected-that-this-message-does-not-meet-IPv6-sending-guidelines-regarding-PTR-records
• https://support.plesk.com/hc/en-us/articles/213388269-Unable-to-send-mail-using-STARTTLS-authentication-4-7-0-TLS-not-available-due-to-local-problem
• https://blog.kruyt.org/postfix-and-tls-encryption/