NIPR.MIL has no MX records.
Question and response
Question
dig NIPR.MIL. MX
Answer
The response has no answer records.
Authority
NIPR.MIL. 60 SOA dns1.NIPR.MIL. hostmaster\@nic.MIL. 37613 21600 3600 2419200 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0