mx1.tuke.sk has no CNAME records.
Question and response
Question
dig mx1.tuke.sk. NS
Answer
The response has no answer records.
Authority
tuke.sk. 5646 SOA a.ns.tuke.sk. hostmaster.tuke.sk. 2026022505 21600 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0