mx01.hornetsecurity.com has no MX records.
Question and response
Question
dig mx01.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