mgw01.ironmountain.com.pl has no MX records.
Question and response
Question
dig mgw01.ironmountain.com.pl. MX
Answer
The response has no answer records.
Authority
ironmountain.com.pl. 900 SOA nusdnsgm01.ironmountain.com. webmaster.ironmountain.com. 185 10800 3600 2592000 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0