mailbox2.ad.tuke.sk has no CNAME records.
Question and response
Question
dig mailbox2.ad.tuke.sk. NS
Answer
The response has no answer records.
Authority
ad.tuke.sk. 2716 SOA dc1.ad.tuke.sk. hostmaster.ad.tuke.sk. 330018 900 600 86400 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0