mailserver.ops-netman.net has no CNAME records.
Question and response
Question
dig mailserver.ops-netman.net. NS
Answer
The response has no answer records.
Authority
ops-netman.net. 4271 SOA ops-netman.net. hostmaster.ops-netman.net. 2016032311 21600 3600 2592000 21600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0