mx2.uni-rostock.de has no CNAME records.
Question and response
Question
dig mx2.uni-rostock.de. NS
Answer
The response has no answer records.
Authority
uni-rostock.de. 900 SOA john.ns.uni-rostock.de. hostmaster.uni-rostock.de. 2012013632 86400 300 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0