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