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What is reverse DNS?
Reverse DNS resolves an address back to a name. For mail servers it is a requirement, not a nicety.
The lookup happens in in-addr.arpa or ip6.arpa, zones derived from the address itself and controlled by whoever owns the range.
Receivers check that the name in the PTR resolves back to the same address — forward-confirmed reverse DNS. If it does not, mail is treated as suspicious.
Both protocols count. A server that may send over IPv6 needs an IPv6 PTR as well.
We manage PTR zones for ranges delegated to our nameservers; for addresses from a provider, the PTR is set at that provider.