iptables -I INPUT 1 -p tcp -s zabbix server IP-address --dport 10050 -j ACCEPT
iptables -I OUTPUT 1 -p tcp -s zabbix server IP-address --dport 10051 -j ACCEPT
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From @rafaeleduardo
iptables -I INPUT 1 -p tcp -s zabbix server IP-address --dport 10050 -j ACCEPT
iptables -I OUTPUT 1 -p tcp -s zabbix server IP-address --dport 10051 -j ACCEPT