intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
Whoisology - Connected database of whois records and whois history
NsLookup.io - A simple and powerful tool for querying DNS records
JSON WHOIS API - Parsed domain WHOIS data from any WHOIS server via RESTful API with metered pay as you pricing.