Good luck getting the average residential ISP to edit rDNS records for you. It's a special thing they will only do in certain circumstances and as a special one-off thing. Automated provisioning tools and customer service are not set up for it. I'm not saying that is a good thing, or satisfactory, but that's the current status quo.
I'm all in favor of ipv6 everywhere but the current reality is that a huge number of organizations presently have v4 only smtpd in their MX records, if you try to do a pure v6 approach you will break delivery to/from something like 65% of the internet.
I'm all in favor of ipv6 everywhere but the current reality is that a huge number of organizations presently have v4 only smtpd in their MX records, if you try to do a pure v6 approach you will break delivery to/from something like 65% of the internet.