mail01.uptime.de has no CNAME records.
Question and response
Question
dig mail01.uptime.de. NS
Answer
The response has no answer records.
Authority
uptime.de. 6000 SOA ns01.uptime.de. hostmaster.uptime.de. 2026022501 10800 3600 1814400 21600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0