ns1.flex1.net has no MX records.
Question and response
Question
dig ns1.flex1.net. MX
Answer
The response has no answer records.
Authority
flex1.net. 900 SOA ns1.flex1.net. noc.flex1.net. 110 900 300 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0