If you get a message like this in your monit log files:
alert handler failed, retry scheduled for next cycle
It’s not immediately obvious what’s causing it. Check the line before, it’s probably what you’re looking for.
In my case I was getting this error
Sendmail error: 553 5.1.7 [email protected]… address incomplete
Which is actually from the mailserver I’d setup in the monitrc file. Make sure you’ve got your settings right, and don’t just look at the one line 🙂