smtp-backup2.portunity.de has no CNAME records.
Question and response
Question
dig smtp-backup2.portunity.de. NS
Answer
The response has no answer records.
Authority
portunity.de. 56 SOA ns.portunity.de. hostmaster.portunity.de. 1771368784 10800 3600 3600000 360
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0