mail.nsentry.de has no CNAME records.
Question and response
Question
dig mail.nsentry.de. NS
Answer
The response has no answer records.
Authority
nsentry.de. 606 SOA ns1.nsentry.de. hostmaster.nsentry.de. 2017013000 86400 7200 3600000 172800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0