A usual email rejection message is becoming more and more frequent. Our system has detected that this message does 550-5.7.1 not meet IPv6 sending guidelines regarding PTR records and 550-5.7.1 authentication. Please review 550-5.7.1.Solution (explanation of the problem is below the solution)

1. Contact your hosting provider to add IPv6 rDNS records. This will not happen probably so go to 2
2. Go to /etc/postfix/main.cf and change inet_protocols = all to inet_protocols = ipv4. In case this line does not exist, simply add it and the issue is resolved.

 

This is happening because of the adoption of IPv6 protocol, but almost nobody offers the ability to add a Reverse DNS entry for an IPv6 address.

The obvious (but not always achievable) solution is to contact your hosting provider in order to add the required IPv6 rDNS records. In most of the cases this will not happen, because the majority of hosting companies do not offer full IPv6 support, thus not offering IPv6 rDNS records. The only companies in my knowledge offering this now are Hetzner and Vultr(I am not affiliated to them, just stating a fact).