Postfix: Our system has detected that this message does not 550-5.7.1 meet IPv6 sending guidelines

Ipv6…nice. But using Ipv6 when sending emails to google can be a pain in the *ss.
It is possible that you get the following message in your postfix logs:

 Our system has detected that this message does not 550-5.7.1 meet IPv6 sending guidelines regarding PTR records and authentication 550-5.7.1

As workaround you can configure postfix to send emails using ipv4 when sending emails to Gmail.
In order to do that, you can perform the following steps:

#vi /etc/postfix/main.cf

Find the rule that starts with transport_maps.
Add ‘hash:/etc/postfix/transport’ to that line. If it doesn’t exist yet, you can add a new line:

transport_maps = hash:/etc/postfix/transport

Save & exit main.cf
Create a transport file:

#vi /etc/postfix/transport

Add the following line:

gmail.com       smtp-ipv4:

Save & exit transport file.
now create the transport file:

# postmap hash:/etc/postfix/transport

Edit the master.cf file, and add the contents below

 # vi /etc/postfix/master.cf

Restart postfix, and you are ready to sending emails to gmail.

# systemctl restart postfix