mail.skyline-bw.com has no MX records.
Question and response
Question
dig mail.skyline-bw.com. MX
Answer
The response has no answer records.
Authority
skyline-bw.com. 86400 SOA a.regfish-ns.net. dns.regfish-ns.net. 1771881253 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0