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.
Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
DNSInspect - DNS is a free online service to check your domain’s servers for common DNS and mail errors and produces a report with explanations how to fix them.
DNS Checker Tool - Check Domain Name System (DNS) records for any website.
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.