intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
SolveDNS - SolveDNS works to check your name servers, a records, and mail servers, SOA records for errors, consistency, efficiency and speed using several DNS tests.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services