I configured the SMTP module. The test is working. The e-mail arrives.
I have a new module with heartbeat. When it is of-line, the error message evoked by the related rule generates a high alert. In the log the the entry is there. SMTP does not react.
It is working again. Why I don't know but who cares?
The behaviour is unstable. Only every now and than a message is sent. No trend to see.
I guess the alert is triggering, you see it under notification but the email does not arrive, correct? If so look at the logs for any error coming from the notification/smtp module. If nothing is showing up, try enabling debug mode for that module and trigger the notification. At least here the smtp notification never failed. Also ensure there are no suppression rules configured. Thanks!
Your advice was right. I had a suppression on SMTP messages per hour instead of suppression on individual rules.