smtp.maxcluster.de has no CNAME records.
Question and response
Question
dig smtp.maxcluster.de. NS
Answer
The response has no answer records.
Authority
maxcluster.de. 3600 SOA nameserver-1-de.maxcluster.net. info.maxcluster.net. 2025112401 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0