DNS4.DISA.MIL has no MX records.
Question and response
Question
dig DNS4.DISA.MIL. MX
Answer
The response has no answer records.
Authority
DISA.MIL. 656 SOA dns1.DISA.MIL. disa\.tinker\.ie\.list\.dci-ii-dns.mail.MIL. 2027415184 10800 1080 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0