smtp.jsn.de has no CNAME records.
Question and response
Question
dig smtp.jsn.de. NS
Answer
The response has no answer records.
Authority
jsn.de. 259 SOA x.ns.joker.com. hostmaster.joker.com. 2025121673 14400 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0