mx.hubapi.com has no MX records.
Question and response
Question
dig mx.hubapi.com. MX
Answer
The response has no answer records.
Authority
hubapi.com. 1800 SOA jerry.ns.cloudflare.com. dns.cloudflare.com. 2397377971 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0