mx-cluster-ca01.hornetsecurity.com has no MX records.
Question and response
Question
dig mx-cluster-ca01.hornetsecurity.com. MX
Answer
The response has no answer records.
Authority
hornetsecurity.com. 1800 SOA hank.ns.cloudflare.com. dns.cloudflare.com. 2397088619 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0