hs2._domainkey.notify.axcient.com has no MX records.
Question and response
Question
dig notify-axcient-com.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. 2047468430 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0