spf.flexmail.net has no MX records.
Question and response
Question
dig spf.flexmail.net. MX
Answer
The response has no answer records.
Authority
flexmail.net. 3600 SOA ns3.combell.net. hostmaster.flexmail.net. 1764922866 10800 3600 604800 40000
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0