mx2.rus.uni-stuttgart.de has no CNAME records.
Question and response
Question
dig mx2.rus.uni-stuttgart.de. NS
Answer
The response has no answer records.
Authority
rus.uni-stuttgart.de. 579 SOA dns-hm0.rus.uni-stuttgart.de. dns-support.rus.uni-stuttgart.de. 2026022006 3600 900 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0