Reverse DNS (PTR record)
The DNS record that maps an IP address back to a hostname. Mail servers check it, and many refuse mail from an address without one.
Forward DNS turns a name into an address; reverse DNS turns an address back into a name. The record type is PTR and it lives in a special zone: 31.24.249.0 is looked up as 0.249.24.31.in-addr.arpa, IPv6 addresses as a long name under ip6.arpa.
Reverse DNS belongs to whoever holds the address block, not to the domain owner, so you set it with your hosting provider rather than at your registrar. On a NovaNode VPS you set it yourself in the control panel; the forward record must exist first.
Mail servers care because a PTR that matches the sending host's name (forward-confirmed reverse DNS) is a cheap sign that the sender is a real server and not a compromised home connection. Without it, a lot of mail from a new server is refused outright or lands in spam. A hostname like mail.example.lu with matching A, AAAA and PTR records is the baseline for any server that sends mail.