How to fix Postfix '550-5.7.1 does not 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 will 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 a 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 the transport file. Now create the transport map: # 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 send emails to Gmail. # systemctl restart postfix