Reverse DNS is a way of associating an IP address with its domain name.
The reverse DNS identifier is contained in the PTR portion of the IP Zone File.
The IP Zone File contains all the different ways that your IP and domain name can be associated; each association serves a different need.
AOL does require that all connecting Mail Transfer Agents have established reverse DNS,
regardless of whether it matches the domain.
Reverse DNS must be in the form of a fully-qualified domain name.
Reverse DNS containing in-addr.arpa are not acceptable,
as these are merely placeholders for a valid PTR record. Reverse DNS consisting only of IP addresses are also not acceptable,
as they do not correctly establish the relationship between domain and IP address.