mailman.fireeye.com has no TXT records.
Question and response
Question
dig mailman.fireeye.com. TXT
Answer
The response has no answer records.
Authority
fireeye.com. 1800 SOA bonnie.ns.cloudflare.com. dns.cloudflare.com. 2397531943 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0