Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
UptimeRobot - Free Website Uptime Monitoring
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
Uptime Kuma - A fancy self-hosted monitoring tool.
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.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).