mail.skyline-bw.com has no CNAME records.
Question and response
Question
dig mail.skyline-bw.com. NS
Answer
The response has no answer records.
Authority
skyline-bw.com. 10800 SOA a.regfish-ns.net. dns.regfish-ns.net. 1772831664 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0