Fixes email sending notifications so they are checked if they are handled.

src/Umbraco.Core/Notifications/SendEmailNotification.cs

@@ -7,5 +7,15 @@ namespace Umbraco.Cms.Core.Notifications
         public SendEmailNotification(NotificationEmailModel message) => Message = message;
 
         public NotificationEmailModel Message { get; }
+
+        /// <summary>
+        /// Call to tell Umbraco that the email sending is handled.
+        /// </summary>
+        public void HandleEmail() => IsHandled = true;
+
+        /// <summary>
+        /// Returns true if the email sending is handled.
+        /// </summary>
+        public bool IsHandled { get; private set; }
     }
 }