I recently received an error when sending an email to google:

2607:f8b0:4003:0c17:0000:0000:0000:001b failed after I sent the message.
Remote host said: 550-5.7.1 [2607:5201:1110:3110:ec4:7aff:fe7d:88f3] Our system has detected that
550-5.7.1 this message does not meet IPv6 sending guidelines regarding PTR
550-5.7.1 records and authentication. Please review
550-5.7.1 https://support.google.com/mail/?p=IPv6AuthError for more information
550 5.7.1 . t203si2123379oib.437 - gsmtp

The fix is to disable IPv6 in CentOS and postfix

Resources:
https://wiki.centos.org/FAQ/CentOS6#head-d47139912868bcb9d754441ecb6a8a10d41781df
https://www.thegeekdiary.com/centos-rhel-7-how-to-disable-ipv6/
https://serverfault.com/questions/732187/sendmail-can-not-deliver-to-gmail-ipv6-sending-guidelines-regarding-ptr-record

For Plesk:
https://support.plesk.com/hc/en-us/articles/115002922789-How-to-disable-IPv6-addresses-on-a-Plesk-server-