ns2.sitel.net.pl has no MX records.
Question and response
Question
dig ns2.sitel.net.pl. MX
Answer
The response has no answer records.
Authority
sitel.net.pl. 3600 SOA ns1.e-strefa.net. admin.sitel.net.pl. 2025121801 300 1800 3600000 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0