_spf.rbg.tum.de has no CNAME records.
Question and response
Question
dig _spf.rbg.tum.de. NS
Answer
The response has no answer records.
Authority
rbg.tum.de. 3600 SOA tuminfo1.rbg.tum.de. rbg-system.rbg.tum.de. 45880 86400 3600 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0