It really depends. In our case, Gmail rejects our IPv6, but accepts IPv4. The IPs are from university network and they don't have any history other than being used by us. SPF, DKIM and PTR records are in place.

> Our system has detected that this message is likely suspicious due to the very low reputation of sending domain. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information.

This message is extra confusing because mail from said "sending domain" is accepted by IPv4.