ns4.combell.net has no MX records.
Question and response
Question
dig ns4.combell.net. MX
Answer
The response has no answer records.
Authority
combell.net. 3600 SOA ns3.combell.net. hostmaster.combell.net. 1755814554 10800 3600 604800 40000
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0