mailing.theory7.net has no CNAME records.
Question and response
Question
dig mailing.theory7.net. NS
Answer
The response has no answer records.
Authority
theory7.net. 3302 SOA dns.theory7.net. hostmaster.theory7.net. 2026020504 10800 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0