mx.post.pl has no MX records.
Question and response
Question
dig mx.post.pl. MX
Answer
The response has no answer records.
Authority
mx.post.pl. 3600 SOA ns1.home-whs.pl. info.hbscloud.com. 7592364 14400 7200 2419200 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0