ncopenbook.gov has no MX records.
Question and response
Question
dig ncopenbook.gov. MX
Answer
The response has no answer records.
Authority
gov. 300 SOA a.ns.gov. dns.cloudflare.com. 1771850705 3600 900 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0