smtp.doc.gov has no MX records.
Question and response
Question
dig smtp.doc.gov. MX
Answer
The response has no answer records.
Authority
doc.gov. 1800 SOA cecelia.ns.cloudflare.com. dns.cloudflare.com. 2397266244 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0