This question is mostly out of curiosity as I have now updated the SPF record to include the ipv6 address.
I have a server and I tested its php email sending ability by sending myself an email from it. I had already set an SPF record which allows the server's ipv4 address to send email but marks others as suspicious.
The test email was received but marked as spam with the header saying something like:
google.com: domain of transitioning firstname.lastname@example.org does not designate [my server's ipv6 address] as permitted sender) client-ip=[my server's ipv6 address]
So was gmail only looking at my server's ipv6 to see if is allowed to send from my domain? I've since updated the dns record but it would be reassuring to know if this was the cause.
(the test mail was send from a wordpress contact form so had all the appropriate headers etc)