mailman.fireeye.com has no MX records.
Question and response
Question
dig mailman.fireeye.com. MX
Answer
The response has no answer records.
Authority
fireeye.com. 1800 SOA bonnie.ns.cloudflare.com. dns.cloudflare.com. 2397163774 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0