mx1.plusnet.de has no CNAME records.
Question and response
Question
dig mx1.plusnet.de. NS
Answer
The response has no answer records.
Authority
plusnet.de. 2510 SOA ns1.plusnet.de. hostmaster.plusnet.de. 2026021002 36000 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0