smtp-01.netwebmail.de has no CNAME records.
Question and response
Question
dig smtp-01.netwebmail.de. NS
Answer
The response has no answer records.
Authority
netwebmail.de. 900 SOA ns1.routing.net. hostmaster.netwebmail.de. 2020022802 86400 7200 2419200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0