Gmail rejects emails because it does not meet IPv6 sending guidelines.
SYMPTOMS

Every email sent from a terminal of a Linux server to Gmail fails to be delivered. Postfix is used for sending emails.

 
There is the following error stack in the /var/log/maillog file.

[root@dbpilot ~]# cat /var/log/maillog

Apr 17 10:41:38 ns7000905 postfix/pickup[38640]: 16E10: uid=0 from=
Apr 17 10:41:38 ns7000905 postfix/cleanup[38650]: 16E10: message-id=<20180417144138.16E1046B825C6@dbpilot.net>
Apr 17 10:41:38 ns7000905 postfix/qmgr[38641]: 16E10: from=, size=468, nrcpt=1 (queue active)
Apr 17 10:41:38 ns7000905 postfix/smtp[38652]: 16E10: to=, relay=gmail-smtp-in.l.google.com
[2607:]: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:] 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]: 4785C4: message-id=<20180417144138.4785C46B825C7@dbpilot.net>
Apr 17 10:41:38 ns7000905 postfix/bounce[38653]: 16E104: sender non-delivery notification: 4785C46B825C7
Apr 17 10:41:38 ns7000905 postfix/qmgr[38641]: 4785C4: from=<>, size=3063, nrcpt=1 (queue active)
Apr 17 10:41:38 ns7000905 postfix/qmgr[38641]: 16E104: removed
Apr 17 10:41:38 ns7000905 postfix/local[38654]: 4785C4: 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

CAUSE

Gmail Guidelines for IPv6 are not met.

WORKAROUND

By editing the /etc/postfix/main.cf file, configure Postfix to use IPv4 addresses only.


################
## Mail Version
################

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

################
## Current value of the inet_protocols parameter
################

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

################
## Required value of the inet_protocols parameter
################

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

################
## Restart postfix service 
################

[root@dbpilot ~]# systemctl restart postfix
NOTE

Check your Gmail spam folder if you do not receive any emails after applying the workaround.

Written At
31 OCT 201812:00
CentOS Release
7.4 x64
Postfix Version
2.10.1