OUR SYSTEM HAS DETECTED THAT THIS MESSAGE DOES NOT MEET IPV6 [GMAIL]

When an email is sent from the server to a Gmail account it fails to deliver.
The postfix is used as Mail Transfer Agent to deliver emails.

In the /var/log/maillog the following error appears

[root@dbpilot ~]# cat /var/log/maillog
Apr 17 10:41:38 ns7000905 postfix/pickup[38640]: 16E1046B825C6: uid=0 from=
Apr 17 10:41:38 ns7000905 postfix/cleanup[38650]: 16E1046B825C6: message-id=<20180417144138.16E1046B825C6@dbpilot.net>
Apr 17 10:41:38 ns7000905 postfix/qmgr[38641]: 16E1046B825C6: from=, size=468, nrcpt=1 (queue active)
Apr 17 10:41:38 ns7000905 postfix/smtp[38652]: 16E1046B825C6: to=, relay=gmail-smtp-in.l.google.com[2607:f8b0:400d:c08::1a]:25, delay=0.2, delays=0.01/0/0.08/0.11, dsn=5.7.1, status=bounced (host gmail-smtp-in.l.google.com[2607:f8b0:400d:c08::1a] said: 550-5.7.1 [2607:5300:60:cc8c::] 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. Please review 550-5.7.1  https://support.google.com/mail/?p=IPv6AuthError for more information 550 5.7.1 . m15si10283441qtc.60 - gsmtp (in reply to end of DATA command))
Apr 17 10:41:38 ns7000905 postfix/cleanup[38650]: 4785C46B825C7: message-id=<20180417144138.4785C46B825C7@dbpilot.net>
Apr 17 10:41:38 ns7000905 postfix/bounce[38653]: 16E1046B825C6: sender non-delivery notification: 4785C46B825C7
Apr 17 10:41:38 ns7000905 postfix/qmgr[38641]: 4785C46B825C7: from=<>, size=3063, nrcpt=1 (queue active)
Apr 17 10:41:38 ns7000905 postfix/qmgr[38641]: 16E1046B825C6: removed
Apr 17 10:41:38 ns7000905 postfix/local[38654]: 4785C46B825C7: to=, relay=local, delay=0.01, delays=0/0.01/0/0, dsn=2.0.0, status=sent (delivered to mailbox)
Apr 17 10:41:38 ns7000905 postfix/qmgr[38641]: 4785C46B825C7: removed

Gmail Guidelines for IPv6 are not met

################
## Version
################
[root@dbpilot ~]# postconf -d |grep mail_version
mail_version = 2.10.1
milter_macro_v = $mail_name $mail_version

################
## Change this
################
[root@dbpilot ~]# egrep inet_protocol /etc/postfix/main.cf
inet_protocols = all

################
## To this
################
[root@dbpilot ~]# egrep inet_protocol /etc/postfix/main.cf
inet_protocols = ipv4

################
## Restart postfix service 
################
[root@dbpilot ~]# systemctl restart postfix