mail.blueserver.eu has no CNAME records.
Question and response
Question
dig mail.blueserver.eu. NS
Answer
The response has no answer records.
Authority
blueserver.eu. 10800 SOA ns1.bmsdns.de. hostmaster.bmsdns.de. 2024041250 14400 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0