mail1.net.de has no CNAME records.
Question and response
Question
dig mail1.net.de. NS
Answer
The response has no answer records.
Authority
net.de. 434 SOA ns.pop-hannover.de. hostmaster.net.de. 2026030301 21600 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0