mail01.vnet.sk has no CNAME records.
Question and response
Question
dig mail01.vnet.sk. NS
Answer
The response has no answer records.
Authority
vnet.sk. 2314 SOA ns.vnet.sk. hostmaster.vnet.sk. 2026030401 3600 900 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0