ns3.impervadns.net has no MX records.
Question and response
Question
dig ns3.impervadns.net. MX
Answer
The response has no answer records.
Authority
impervadns.net. 900 SOA ns1.impervadns.net. hostmaster.incapsula.com. 3001 86400 7200 3600000 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0