Maybe the volume is turned down or muted. Therefore, you need to check if that is the case. Alternatively, we suggest checking if Do Not Disturb is activated. If it is, disable it, and you should be notified of the arrival of your next email. Lastly, check if notification settings are disabled on an app basis through Android settings. They can be disabled with a long press on the notification, but there is no way to bring it back the same way.