_spf.nameserver.sk has no MX records.
Question and response
Question
dig _spf.nameserver.sk. MX
Answer
The response has no answer records.
Authority
nameserver.sk. 300 SOA ns1.dnsbackup.net. root.dnsbackup.net. 2026021901 36000 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0