Postfix : Fixing email sending issues to Gmail due to IPv6

If you are unable to send emails to Gmail addresses from your server and you encounter the following error, it is likely a configuration issue if you have both IPv4 and IPv6:

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.

To resolve this issue, it's quite simple.

1 - Connect to your server via SSH and use the following command to check the connection protocol used by Postfix:

postconf inet_protocols

If you get "inet_protocols = all", you need to modify the Postfix configuration file to use only IPv4.

2 - To do this, use the following command to edit the configuration file:

nano /etc/postfix/main.cf

3 - Then, modify the following line:

inet_protocols = all

and replace it with the following:

inet_protocols = ipv4

4 - Finally, restart Postfix using the following command:

service postfix restart

There you go, your server is now properly configured to send emails using IPv4.