_validation.heysender.com has no CNAME records.
Question and response
Question
dig _validation.heysender.com. NS
Answer
The response has no answer records.
Authority
heysender.com. 266 SOA ns1.curanet.dk. hostmaster.curanet.dk. 2025111401 14400 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0