#Ipv6

It seems that Google’s anti spam measurement for IPv6 is basically checking if the IP has a reverse DNS. If not, reject the e-mail:

This is the mail system at host rootspirit.com.

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

<xxx@gmail.com>: host
gmail-smtp-in.l.google.com[2a00:1450:4013:c01::1b] said: 550-5.7.1
[2001:1af8:3100:a00a:21::1010 12] Our system has detected that
550-5.7.1 this message is likely unsolicited mail. To reduce the amount
of spam 550-5.7.1 sent to Gmail, this message has been blocked.
Please visit 550-5.7.1 http://support.google.com/mail/bin/answer.py?hl=en&answer=188131 for 550 5.7.1 more information. df5si15766518wjb.42 - gsmtp (in reply to end of DATA command)

There is no difference if the mail is a one word e-mail or a wall of text that crits over 9000.