nussmtp01.ironmountain.com has no MX records.
Question and response
Question
dig nussmtp01.ironmountain.com. MX
Answer
The response has no answer records.
Authority
ironmountain.com. 900 SOA nusdnsgm01.ironmountain.com. please_set_email.absolutely.nowhere. 2005443537 10800 1080 1728000 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0