mailing.smcdn.pl has no MX records.
Question and response
Question
dig mailing.smcdn.pl. MX
Answer
The response has no answer records.
Authority
smcdn.pl. 1800 SOA cloe.ns.cloudflare.com. dns.cloudflare.com. 2396952200 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0