cust1.mta.4web.pl has no MX records.
Question and response
Question
dig cust1.mta.4web.pl. MX
Answer
The response has no answer records.
Authority
4web.pl. 86400 SOA ns1.4web.pl. dnsadmin.4web.pl. 2005041155 7200 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0