Hey Hari,
Whenever we attempt to deliver an event to your webhook endpoint we expect a 2xx response code to indicate that you handled it properly on your end. If you respond something else, we assume that you didn't get the event which means that we will try to deliver it again every hour for up to three days before giving up on that event completely.
If your server is down longer than 3 days, some of those events won't be retried anymore. In that case, you'll need to handle those pending events yourself. The idea would be to use the List Events API [1] to loop through all the events. For each event, you can check the `pending_webhooks` property to see if that event has been successfully delivered to all endpoints or not. You can assume that if this property is set to 1, this is one of the events you haven't processed and you can then retrieve it and handle it yourself.
Hope this helps!
Remi