mail3.pvt.net has no CNAME records.
Question and response
Question
dig mail3.pvt.net. NS
Answer
The response has no answer records.
Authority
pvt.net. 8627 SOA dns.nextra.cz. hostmaster.cz.net. 2015081003 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0