mail.ze.tum.de has no CNAME records.
Question and response
Question
dig mail.ze.tum.de. NS
Answer
The response has no answer records.
Authority
ze.tum.de. 1427 SOA w3projns.ze.tum.de. schmidt.ze.tum.de. 2026030608 7200 3600 3628800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0