_spf.stuba.sk has no CNAME records.
Question and response
Question
dig _spf.stuba.sk. NS
Answer
The response has no answer records.
Authority
stuba.sk. 4141 SOA ns.cvt.stuba.sk. postmaster.stuba.sk. 2026022300 7200 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0