mail.dnsguard.pub has no MX records.
Question and response
Question
dig mail.dnsguard.pub. MX
Answer
The response has no answer records.
Authority
dnsguard.pub. 1800 SOA art.ns.cloudflare.com. dns.cloudflare.com. 2397189331 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0