nagios-ext.mon.inetu.net has no MX records.
Question and response
Question
dig nagios-ext.mon.inetu.net. MX
Answer
The response has no answer records.
Authority
mon.inetu.net. 3600 SOA ns3.inetu.net. root.ns3.inetu.net. 2018042700 3600 300 86400 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0