_smtp.udag.de has no CNAME records.
Question and response
Question
dig _smtp.udag.de. NS
Answer
The response has no answer records.
Authority
udag.de. 3600 SOA ns.udag.de. hostmaster.united-domains.de. 2024093019 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0