ns4.fortinet.com has no MX records.
Question and response
Question
dig ns4.fortinet.com. MX
Answer
The response has no answer records.
Authority
fortinet.com. 300 SOA ns3.fortinet.com. mis.fortinet.com. 1 604800 7200 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0