e.mx.sonic.net has no CNAME records.
Question and response
Question
dig e.mx.sonic.net. NS
Answer
The response has no answer records.
Authority
mx.sonic.net. 300 SOA ns1.sonic.net. hostmaster.sonic.net. 2025102701 3600 900 1209600 28800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0