nscluster.sk has no MX records.
Question and response
Question
dig nscluster.sk. MX
Answer
The response has no answer records.
Authority
nscluster.sk. 901 SOA ns.nscluster.eu. hostmaster.nscluster.eu. 2025111804 7200 3600 1209600 901
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0