Contents
1. whois
Either I’d left DNS3 configured on my domain, pointing to an IP address I no longer controlled. Or I’d configured Nameservers4 on my domain to point to a DNS provider but hadn’t configured DNS.
The moral of the story: Be careful with your domain names and DNS records. Ensure you set the nameservers on your domains intentionally, and you control the DNS for all of your domains. Also remember to remove old subdomains and DNS records that are no longer in use or needed.