mta.mail-u.cloud has no CNAME records.
Question and response
Question
dig mta.mail-u.cloud. NS
Answer
The response has no answer records.
Authority
mail-u.cloud. 2107 SOA ns.udag.de. hostmaster.united-domains.de. 2024091364 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0