d.mail.sonic.net has no CNAME records.
Question and response
Question
dig d.mail.sonic.net. NS
Answer
The response has no answer records.
Authority
sonic.net. 10800 SOA a.auth-ns.sonic.net. hostmaster.sonic.net. 2026022501 10800 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0