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. 355 SOA dns1.DISA.mil. disa\.tinker\.ie\.list\.dci-ii-dns.mail.mil. 2027415447 10800 1080 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0