host gmail-smtp-in.l.google.com[2a00:1450:400c:c05::1b] said: 550-5.7.1 [2001:db8:8e3f:43c7::12      16] Our system has detected that this message does not meet IPv6 sending guidelines regarding PTR records and authentication. Please review https://support.google.com/mail/?p=ipv6_authentication_error for more information. e9si11530274wjb.156 - gsmtp (in reply to end of DATA command)