Once you obtain a domain from a commercial registrar, it may be a good idea to change over your rDNS to your new domain name, especially if you plan on sending and receiving e-mail through your server.
Before doing so, make sure both the A and AAAA records point to your server (Linode supports both IPv4 and IPv6 so you might as well use both).
To do so, first click the name of your server in the legacy manager.linode.com control panel.
Next, click the “Remote Access” tab.
On the page that appears, under “Public IPs”, look for “Reverse DNS” and click it.
Enter your fully qualified domain name i.e. www.example.com or mail.example.com (without http or https) and click Look up. When it finds a match it will prompt to change the rDNS records for your IPv4 and IPv6 addresses to the domain name.