Reverse DNS
Internet Standard. Method for systems to resolve an IP address from a domain name. RFC 5855 defines this standard for IPv4 addresses. RFC 8501 defines this standard for IPv6 addresses.
The primary purpose of the Domain Name System (DNS) is to map a domain to an IP address. Some authentication methods want to verify that an IP address belongs to a specific domain. Using Reverse DNS, a recipient server can verify that the given sender's IP address belongs to the domain given in the sender's email address.
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Reverse DNS, not IP whitelabel
You might encounter the term IP whitelabel. The term reverse DNS replaces that term and eliminates any potential racial or cultural connotations associated with the earlier term.