you can try checking "Logs"->"System Events"->"Notifications", but please bear in mind that entries there are purged after 30 days (30 is the default setting, the actual setting in your setup might even be shorter), and that through summarization a relation between trigger/notification and sent email might not be exactly possible.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.