ns3.exoscale.net has no MX records.
Question and response
Question
dig ns3.exoscale.net. MX
Answer
The response has no answer records.
Authority
exoscale.net. 300 SOA ns2.exoscale.net. hostmaster.exoscale.net. 1388212207 7200 1800 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0