Beginning with Android 6, a doze mode feature was included which optimizes memory and battery life by restricting app background usage when the device is not in use. This almost always affects Aqua Mail's ability to synchronize mail in the background.

The app will notify you via pop-up or in Settings > Mail, receiving if it detects it is being disabled from running.

You can also adjust doze mode from your Android settings. Doze Mode is usually described as battery optimization there. The option to turn it off is located in your Android Settings > Apps > Aqua Mail's app page > Battery. The setting itself could be listed as do not optimize, allow background activity, unmonitored or similar.