mx.posta.telekom.sk has no CNAME records.
Question and response
Question
dig mx.posta.telekom.sk. NS
Answer
The response has no answer records.
Authority
telekom.sk. 3830 SOA ns.telecom.sk. hostmaster.telecom.sk. 2026022620 28800 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0