The PTR record is an A record opposite. While the A record points a domain name to an IP address, the PTR record resolves the IP address to a domain/hostname. For the reverse DNS (Domain Name System) lookup search PTR records are used. The corresponding domain / hostname can be obtained by using the IP address as a result of the lookup. An A record should exist for every PTR record.
Using a reverse DNS setup for a mail server is a best solution. Before accepting messages originating from your mail server, some external mail exchange servers make reverse DNS lookups.
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