mx.dbvonline.de has no CNAME records.
Question and response
Question
dig mx.dbvonline.de. NS
Answer
The response has no answer records.
Authority
dbvonline.de. 2988 SOA dns.iwelt-ag.net. domainmaster.iwelt-ag.net. 2025051402 43200 3600 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0