ns2.onapp.ip-only.net has no MX records.
Question and response
Question
dig ns2.onapp.ip-only.net. MX
Answer
The response has no answer records.
Authority
onapp.ip-only.net. 1200 SOA ns1.onapp.ip-only.net. johan.carlsson.ip-only.se. 24 1200 180 1209600 1200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0