nipr.mil has no MX records.
Question and response
Question
dig nipr.mil. MX
Answer
The response has no answer records.
Authority
nipr.mil. 3600 SOA dns1.nipr.mil. hostmaster\@nic.mil. 37607 21600 3600 2419200 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0