mx-ex.sgh.waw.pl has no CNAME records.
Question and response
Question
dig mx-ex.sgh.waw.pl. NS
Answer
The response has no answer records.
Authority
sgh.waw.pl. 3462 SOA hermes.sgh.waw.pl. hostmaster.sgh.waw.pl. 2026030601 43200 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0