SMTP error from remote mail server after end of data: host gmail-smtp-in.l.google.com [2607:f8b0:400d:c07::1b]: 550-5.7.1 [2607:5300:100:200::1b10] Our system has detected that this message 550-5.7.1 does not meet IPv6 sending guidelines regarding PTR records and 550-5.7...