Improve deliverability to Google mail users (Emails to gmail only over IPv4)

I had some problems with deliverability to gmail and G-Suite email addresses for a while. It turned out that disabling IPv6 for sending to Google's mailservers fixed this problem (which is weird, because the address wasn't blacklisted and a PTR record was set up). Usually, I would not tell you to disable IPv6. But with this trick, you will only disable it for mail to google, not for others.

Open the master.cf:

Code:

nano /etc/postfix/master.cf

Add this at the end:

Code:

smtp-ipv4     unix  -       -       -       -       -       smtp
 -o inet_protocols=ipv4

Now open the main.cf:

Code:

nano /etc/postfix/main.cf

Add

Code:

hash:/etc/postfix/transport

to the setting

Code:

transport_maps =

The line looks like this for me:

Code:

transport_maps = hash:/etc/postfix/transport, hash:/var/lib/mailman/data/transport-mailman, proxy:mysql:/etc/postfix/mysql-virtual_transports.cf

Now open the transport file:

Code:

nano /etc/postfix/transport

And add this:

Code:

gmail.com smtp-ipv4:
google.com smtp-ipv4:
*.google.com smtp-ipv4:
googlemail.com smtp-ipv4:
*.googlemail.com smtp-ipv4:

Run the following commands to apply the changes:

Code:

postmap /etc/postfix/transport
systemctl restart postfix