mx01.t.ht.hr has no CNAME records.
Question and response
Question
dig mx01.t.ht.hr. NS
Answer
The response has no answer records.
Authority
t.ht.hr. 3164 SOA dns1.ht.hr. dns.t.ht.hr. 2602230002 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0