I can telnet into the mail server from the outside. So firewall is OK and forwarding the ports. And the server is running. I then tried wormly.com to run a few tests, and seems something is amiss in the users I'm guessing. Results of wormly sending an email:. Source: almost 3 years ago

Try https://app.freshping.io/ instead, think is more reliable than uptimerobot. Or if you can run a custom script (with crontab) https://healthchecks.io/ is the way to go, here's an example script https://gist.github.com/1activegeek/9a932346d85d20123eb3a60d3b03a095. Source: almost 2 years ago

There are others. LogRocket does this for frontend monitoring, and there are also Freshping, Uptime, Logicmonitor,, all of which should tell you that the health of applications is really important! - Source: dev.to / almost 2 years ago