mx1.work.de has no CNAME records.
Question and response
Question
dig mx1.work.de. NS
Answer
The response has no answer records.
Authority
work.de. 3514 SOA ns.work.de. hostmaster.work.de. 2026021900 28800 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0