mx11.swan.sk has no CNAME records.
Question and response
Question
dig mx11.swan.sk. NS
Answer
The response has no answer records.
Authority
swan.sk. 1389 SOA auth1.ns.swan.sk. hostmaster.swan.sk. 2026030601 86400 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0