mail.stelin.net has no CNAME records.
Question and response
Question
dig mail.stelin.net. NS
Answer
The response has no answer records.
Authority
stelin.net. 559 SOA root-dns.netcup.net. dnsadmin.netcup.net. 2026021670 28800 7200 1209600 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0