Email Notification is NOT SENT when Alert is generated

Hi!
We have noticed from few days that email alert notification is not sent from Cloud Platform when an alert is generated.
Even if ALL kind of alert should generate an email, as per our settings.
In the past email was sent for any kind of error, but starting from (mayby) 1 week ago, this is not happens any more.

I have also check the mail setting and sent a test e-mail with success.
But real ALERT are not sent anymore.
What can be tha cause ?