I tried to send an e-mail from within my Linux CentOS server command line to an [email protected]

And i got an delivery failure e-mail in return:

From: [email protected] (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender

<[email protected]>: host gmail-smtp-in.l.google.com[2607:f8b0:4003:c11::1a]
said: 550-5.7.1 [2604:4600:c07:****::2] 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 . n74si12569917ota.181 - gsmtp (in reply to end of DATA command)
2604:4600:c07:****::2 being an IPv6 assigned to my Linux server. I do not use IPv6, but somehow my mail server (postfix) is using it so the Google verify if my IPv6 has reverse DNS record (called PTR), my server provider do not have any interface to edit IPv6 PTR records.

Here is how it got fixed:

open /etc/postfix/main.cf and find inside:

make sure "inet_protocols" are set to ipv4:
inet_protocols = ipv4

save and apply postfix configuration:
postfix reload

service postfix restart

service postfix status

if one is using sendmail instead of postfix, i think this may help: https://serverfault.com/questions/512615/how-to-stop-sendmail-sending-mail-from-ipv6-instead-of-ipv4