intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
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.
NsLookup.io - A simple and powerful tool for querying DNS records
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.
CheckNameserver.com - Nameserver Checker Tools