mailcheckout03.agenturserver.de has no CNAME records.
Question and response
Question
dig mailcheckout03.agenturserver.de. NS
Answer
The response has no answer records.
Authority
agenturserver.de. 874 SOA ns01.agenturserver.de. support.mittwald.de. 1755875478 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0