I have faced an issue for one of my client. Their emails are found as rejected by google and gave the NDR as follows
mx.google.com rejected your message to the following email addresses:
Your message wasn’t delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept email from certain senders, or another restriction may be preventing delivery.
mx.google.com gave this error:
[IPV6 Address] Our system has detected that this message does not meet IPv6 sending guidelines regarding PTR records and authentication. Please review https://support.google.com/mail/?p=ipv6_authentication_error for more information. j9si5063852wjt.128 – gsmtp
Diagnostic information for administrators:
Generating server: exch01.domain.local
Remote Server returned ‘550-5.7.1 [2002:d925:6c3::d925:6c3] Our system has detected that this message 550-5.7.1 does not meet IPv6 sending guidelines regarding PTR records and 550-5.7.1 authentication. Please review 550-5.7.1 https://support.google.com/mail/?p=ipv6_authentication_error for more 550 5.7.1 information. j9si5063852wjt.128 – gsmtp’
Go to the portal http://whatismyv6.com/ and find the IPV6 address. Create a AAAA record in your public DNS for ‘mail’ record with value as the IPV6 address.
That’s it !