MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
EasyCron - Get frustrated with Cron on your server? Hosting limits your Cron use?
Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
crontab guru - The quick and simple editor for cron schedule expressions by Cronitor
intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
Cronitor - Monitor cron jobs, micro-services, daemons and almost anything else, no setup required. Easier cron troubleshooting and no more silent failures.