zabbix.net has no MX records.
Question and response
Question
dig zabbix.net. MX
Answer
The response has no answer records.
Authority
zabbix.net. 1800 SOA ines.ns.cloudflare.com. dns.cloudflare.com. 2397092384 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0