dnsismagic

It’s always DNS.

Try dnsismagic.com, cloudflare.com NS, or check a mail domain, or analyse a message's headers.

From a terminal

curl dnsismagic.com/mx/example.com
curl dnsismagic.com/mail/example.com
curl 'dnsismagic.com/lookup?name=example.com&type=NS&f=json'

Add f=json for JSON, f=html for the page. No scheme needed — curl sends those over http and they are answered there rather than redirected. https:// works the same.