hs01b.dkim.hubspotemail.net has no MX records.
Question and response
Question
dig hs01b.dkim.hubspotemail.net. MX
Answer
The response has no answer records.
Authority
hs01b.dkim.hubspotemail.net. 180 SOA ns10.digicertdns.com. dns.digicertdns.com. 2047904548 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0