mail5.server.csl.de has no CNAME records.
Question and response
Question
dig mail5.server.csl.de. NS
Answer
The response has no answer records.
Authority
csl.de. 239 SOA a.ns.nrw.net. hostmaster.joker.com. 2026030652 14400 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0