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What is a PTR record?
PTR records are used for reverse DNS: from an IP address back to the associated hostname.
A PTR lives in a special zone derived from the address — in-addr.arpa for IPv4, ip6.arpa for IPv6 — and can only be set by whoever controls that address range.
For mail this is not optional. Receivers check that the sending address has a PTR, and that the name in the PTR resolves back to the same address. If that circle does not close, mail is delayed or refused.
Both protocols need it. A server sending over IPv6 needs a valid IPv6 PTR, not only the IPv4 one.
We manage PTR zones for ranges that are delegated to our nameservers, next to the forward zones for the same hosts.