Alert notify by email not work

Both the notification channel send test and invite email is work and I did received email.
But when the alert status change to ‘alerting’, I didn’t received the alert email.
The log said ‘lvl=info msg=“Sending alert notification to” addresses="[]" singleEmail=true’.
I found no error log, so how can I troubleshoot this problem?

Is it possible for you to see some kind of log for your SMTP gateway? If Grafana has managed to hand over the alert to your mail server and it has failed to deliver it you might see some more information about that in the SMTP gateway’s logs (or, if you’re using a hosted service such as Mailgun or SendGrid, in their dashboard).

Thank you.It’s useful for me to find the root reason.