I get a lots of mails rejected by Gmail with this message:
This is the mail system at host c60.servage.net.
I’m sorry to have to inform you that your message could not
be delivered to one or more recipients. It’s attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<email@example.com>: host gmail-smtp-in.l.google.com[2a00:1450:400c:c06::1a]
said: 550-5.7.1 [2a01:3b0:a00:6:e1fe:ad3e::] Our system has detected that
this message 550-5.7.1 does not meet IPv6 sending guidelines regarding PTR
records and 550-5.7.1 authentication. Please review 550-5.7.1 https://support.google.com/mail/?p=IPv6AuthError for more information 550
5.7.1 . j3si971095wmh.121 – gsmtp (in reply to end of DATA command)