I was sending a message to my mom and got this back in my inbox:
This is the mail system at host mysite.tld.
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
<to@gmail.com>: host
gmail-smtp-in.l.google.com[2a00:1450:4010:c05::1a] said: 550-5.7.1
[2001:983:8610:1:ee68:399c:8145:5d32] 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 . 23-v6si1795890lfy.259 - gsmtp (in reply to end of DATA command)