mail.sldc.net.pl has no MX records.
Question and response
Question
dig mail.sldc.net.pl. MX
Answer
The response has no answer records.
Authority
sldc.net.pl. 2000 SOA ns1.sldc.net.pl. hostmaster.sldc.net.pl. 2026021803 2000 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0