What is reverse DNS (rDNS/PTR)?

A reverse DNS lookup takes an IP address and returns the domain name associated with that IP. A forward DNS lookup does just the opposite.
 
Reverse lookups are commonly used by email servers. Email servers check and see if an email message came from a valid server before bringing it onto their network. Many email servers will reject messages from any server that does not support reverse lookups or from a server that is highly unlikely to be legitimate. Spammers often use IP addresses from hijacked machines, which means there will be no PTR record. Or, they may use dynamically assigned IP addresses that lead to server domains with highly generic names.