host
gmail-smtp-in.l.google.com[2a00:1450:4013:c14::1a] said: 550-5.7.1
[2a00:10d8:10:7:38af:4aff:fe9d:b164] Our system has detected that this
550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR
records 550-5.7.1 and authentication. Please review 550-5.7.1
https://support.google.com/mail/?p=IPv6AuthError for more information 550
5.7.1 . 21si10750600eji.148 - gsmtp (in reply to end of DATA command)