Postfix: connect to gmail-smtp-in.l.google.com [2607:f8b0:400c:c0b::1a]:25: Network is unreachable

Problem:

I am not able to receive email alerts from database server. Because message transfer agent is trying to connect to the Google SMTP via IPv6, which fails.

# tail /var/log/maillog

Jun 12 15:35:10 rac1 postfix/smtp[19725]:connect to 
gmail-smtp-in.l.google.com [2607:f8b0:400c:c0b::1a]:25: 
Network is unreachable

Solution:

Configure Postfix not to use IPv6 by editing /etc/postfix/main.cf with the following:

[root@rac1 ~]# cat /etc/postfix/main.cf | grep inet_protocols
inet_protocols = ipv4

Restart Postfix and check the status:

[root@rac1 ~]# systemctl restart postfix

[root@rac1 ~]# systemctl status  postfix
 ● postfix.service - Postfix Mail Transport Agent
    Loaded: loaded (/usr/lib/systemd/system/postfix.service; enabled; vendor preset: disabled)
    Active: active (running) since Thu 2019-06-13 10:20:48 UTC; 52s ago
   Process: 17431 ExecStop=/usr/sbin/postfix stop (code=exited, status=0/SUCCESS)
   Process: 17449 ExecStart=/usr/sbin/postfix start (code=exited, status=0/SUCCESS)
   Process: 17445 ExecStartPre=/usr/libexec/postfix/chroot-update (code=exited, status=0/SUCCESS)
   Process: 17442 ExecStartPre=/usr/libexec/postfix/aliasesdb (code=exited, status=0/SUCCESS)
  Main PID: 17520 (master)
    Memory: 3.0M
    CGroup: /system.slice/postfix.service
            ├─17520 /usr/libexec/postfix/master -w
            ├─17521 pickup -l -t unix -u
            └─17522 qmgr -l -t unix -u
 Jun 13 10:20:48 rac1.example.com systemd[1]: Starting Postfix Mail Transport Agent…
 Jun 13 10:20:48 rac1.example.com postfix/postfix-script[17518]: starting the Postfix mail system
 Jun 13 10:20:48 rac1.example.com postfix/master[17520]: daemon started -- version 2.10.1, configuration /etc/postfix
 Jun 13 10:20:48 rac1.example.com systemd[1]: Started Postfix Mail Transport Agent