mail.joker.com has no CNAME records.
Question and response
Question
dig mail.joker.com. NS
Answer
The response has no answer records.
Authority
joker.com. 300 SOA x.ns.joker.com. hostmaster.joker.com. 2026030243 14400 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0