postmaster.heysender.com has no CNAME records.
Question and response
Question
dig postmaster.heysender.com. NS
Answer
The response has no answer records.
Authority
heysender.com. 139 SOA ns1.curanet.dk. hostmaster.curanet.dk. 2025111401 14400 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0