I recently ran into issues with the PTR on a mail server and did the lazy/unsustainable move of just disabling IPv6. Wish I had found this post before!
This way you can still receive email from IPv6 servers.
I run an smtp server from home with a static IPv4 address (for which the ISP lets me configure the PTR), but their IPv6 is "beta" and they don't yet support delegation, which is rather frustrating, but different topic.