single-nagios.wisperisp.com has no MX records.
Question and response
Question
dig single-nagios.wisperisp.com. MX
Answer
The response has no answer records.
Authority
wisperisp.com. 3600 SOA dns1.wisperisp.com. hostmaster.wisperisp.com. 2024013597 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0