What is Reverse DNS?
A Reverse DNS lookup is a query that will retrieve the domain associated with a given IP address. It's simply the opposite of the well-known DNS lookup which gives IP addresses associated with a domain.
Reverse DNS lookup is commonly used by email servers to track down malicious messages and spam, as it can identify and verify the domain of an incoming email.
Why is it important?
Reverse DNS lookup is a key element of the security protocol used by email servers to protect recipients from malicious emails.
Not all sending servers are supporting Reverse DNS lookup, but those that don't are more likely to be blocked by receiving servers. This affects negatively your deliverability.
Yet, all major ESPs (Email Service Providers)'s servers do provide Reverse DNS.