MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
Misk.com DNS Lookup - Powerful domain name and ip address tools for DNS Lookup (nslookup), Supports Reverse DNS, FCrDNS...
DNSBL - DNSBL is an online source that shows the detailed blacklist status of IP address, mail server on more than hundred servers located in several parts of the world.
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.