MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.

intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.

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.

NsLookup.io - A simple and powerful tool for querying DNS records

Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.

Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.