b.ns.gov has no MX records.
Question and response
Question
dig b.ns.gov. MX
Answer
The response has no answer records.
Authority
ns.gov. 1800 SOA danica.ns.cloudflare.com. dns.cloudflare.com. 2397247798 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0