Reconnecting to an email server will use extra battery power, so an unstable internet connection will result in extra battery usage. Also, some email servers prematurely terminate idle connections, while the standard says that an idle connection should be kept open for 29 minutes. In these cases you might want to synchronize periodically, for example each hour, instead of continuously. Note that polling frequently (more than every 30-60 minutes) will likely use more battery power than synchronizing always because connecting to the server and comparing the local and remote messages are expensive operations.
If you know that the connection (reception) is bad, it might be worthwhile to decrease the timeout value in the connection-settings tab page to 10–20 seconds, so that the app discovers earlier that no connection is possible, so that the mechanism as described in this FAQ is used faster.
On some devices it is necessary to disable battery optimizations (setup step 3) to keep connections to email servers open. In fact, leaving battery optimizations enabled can result in extra battery usage for all devices, even though this sounds contradictory!
+The Adaptive Battery feature in recent Android versions stops apps running in the background, even when battery optimizations are disabled. When the app is stopped, it will no longer receive new messages. When the app is manually restarted, a full sync is forced, but Android will later stop the app again because Android AI deems this to be better for battery usage.
Most of the battery usage, not considering viewing messages, is due to synchronization (receiving and sending) of messages. So, to reduce the battery usage, set the number of days to synchronize message for to a lower value, especially if there are a lot of recent messages in a folder. Long press a folder name in the folders list and select Edit properties to access this setting.
If you have at least once a day internet connectivity, it is sufficient to synchronize messages just for one day.
Note that you can set the number of days to keep messages for to a higher number than to synchronize messages for. You could for example initially synchronize messages for a large number of days and after this has been completed reduce the number of days to synchronize messages, but leave the number of days to keep messages. After decreasing the number of days to keep messages, you might want to run the cleanup in the miscellaneous settings to remove old files.