mail.servercow.de has no CNAME records.
Question and response
Question
dig mail.servercow.de. NS
Answer
The response has no answer records.
Authority
servercow.de. 145 SOA angus.ns.servercow.de. hostmaster.servercow.de. 2026030501 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0