reverse DNS - Synology
https://community.synology.com/enu/forum/17/post/11450025.10.2021 · Reverse DNS lookup or reverse DNS resolution (rDNS) is the determination of a domain name that is associated with a given IP address. Some companies such as AOL will reject any message sent from a server without rDNS, so you must ensure that you have one. You cannot associate more than one domain name with a single IP address. Your IP address ...