Cannot set the same reverse DNS name for IPV4 and IPV6

I have a VPS that has an IPV4 and an IPV6 address.
I have set both the A and AAAA records to the same value.
As this VPS has a mail server, and some anti-spam measures require that your mail server name must match its reverse DNS, I want to give both the IPV4 and IPV6 reverse DNS the same value.
However, when I set the reverse DNS for the IPV6, it works correctly, but when I try to set the reverse DNS for the IPV4 it does not work claiming that the name is already in use.
Of course it is already in use, but, as I can have both an A and an AAA record with the same name, I expect to be able to have both an IPV4 and an IPV6 address with the same reverse DNS name.
How can this issue be solved, considering the anti-spam measure needs?