mail01.postel.sk has no MX records.
Question and response
Question
dig mail01.postel.sk. MX
Answer
The response has no answer records.
Authority
postel.sk. 86400 SOA auth1.ns.swan.sk. hostmaster.swan.sk. 2011052601 86400 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0