After we implemented Event sink (either synchronous or asynchronous), there are certain situations we dont know why sinks are not firing and how reliable they are in real time situation.
Both synchronous and asynchronous event notifications are reliable under normal conditions.
That is, no notification is skipped on an event.
However, if there is a failure in the store, Exchange server might call your event method multiple times.
For example, Exchange might shut down after three of five event notifications have been invoked.
When Exchange restarts, the three previous event notifications might or might not be invoked a second time.
An event method that fails after being notified will not be notified again for that specific event