07-22-2021, 10:04 AM
The Android Gmail app is using a service in the background where a delivery notification is pushed from the remote end of a connection to google (not gmail.)
Any email client that supports the "IDLE" verb can do something similar without an 3rd party service provided the server supports it (most do.) You will not get immediate notifications during *sleep* because neither the modem nor the WiFi firmware support waking the phone up for network traffic alone (the WiFi card does support WoL so if you can get a WoL packet to the phone when you get an email then you'll get instant notifications during sleep.) I don't know if that will change in the future.
What you can do to get notifications with a latency in the minutes range while still getting decent battery life is use RTCWake instead of sleeping the device. This is often what Android does (maybe the gmail app does it to.) You'll want to tune the sleep period so it's bellow the keep alive period for your mail service or make sure it's awake for long enough to reconnect.
Any email client that supports the "IDLE" verb can do something similar without an 3rd party service provided the server supports it (most do.) You will not get immediate notifications during *sleep* because neither the modem nor the WiFi firmware support waking the phone up for network traffic alone (the WiFi card does support WoL so if you can get a WoL packet to the phone when you get an email then you'll get instant notifications during sleep.) I don't know if that will change in the future.
What you can do to get notifications with a latency in the minutes range while still getting decent battery life is use RTCWake instead of sleeping the device. This is often what Android does (maybe the gmail app does it to.) You'll want to tune the sleep period so it's bellow the keep alive period for your mail service or make sure it's awake for long enough to reconnect.