Just like a domain is assigned an IP address, an IP address can be assigned a domain. This assignment of a domain to an IP address is called reverse mapping.
“IP Address Reverse Mappings” is the address which resolves back you your server, for example to check your server reverse ip mapping you would check the PTR records: https://www.whatsmydns.net/#PTR/18.104.22.168
PTR records are important when configuring settings for email. Some mailservers will not trust mail unless accept a reverse DNS lookup – in one of two forms:
- Many mailservers check that a PTR record even exists
- Strict mailservers do a forward lookup on the name a mailserver introduces itself as (eg mail.myexample.com), verify it is the IP address that is read off the connection, and do a PTR lookup on that IP address to see if it resolves to the same name.