according to the manual, 4th param for net.dns is timeout and 5th param is count of attempts.

$ zabbix_get -s 127.0.0.1 -k net.dns[,zabbix.com,MX,0,85]
1
$ zabbix_get -s 127.0.0.1 -k net.dns[,zabbix.com,MX,-1,85]
1
$ zabbix_get -s 127.0.0.1 -k net.dns[,zabbix.com,MX,-1,-85]
0

a) timeout of 0 doesn't seem to be sane (is it ignored); negative timeout even less so
b) negative attempt count doesn't seem sane either

also...
$ zabbix_get -s 127.0.0.1 -k net.dns[,zabbix.com,MX,zomg]
1