I reopen this post only to clarify the position of pingdom alert service.
I've found the real cause of the problem... a single email used with a script for synchronization activity.
Pingdom was absolutely "ok"... I'm sorry to have created bad publicity to this great monitoring service.
I hope to see soon on Plesk more controls about single domain activity (not only single email stats but also CPU usage for a specific domain and other things useful to found any problems).
It's been more than two years since your post, I hope you or any one who reads my reply can help me on the same issue. What did you mean by "single email used with a script for synchronization activity"? How did you find it and how did you resolve it?