smtp6.heysender.com has no CNAME records.
Question and response
Question
dig smtp6.heysender.com. NS
Answer
The response has no answer records.
Authority
heysender.com. 199 SOA ns1.curanet.dk. hostmaster.curanet.dk. 2026030201 14400 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0