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!
If you can't control your PTR on IPv6, in postfix I recommend using: "smtp_address_preference = ipv4", instead of fully disabling IPv6.
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.
