OPPO's ColorOS may require additional setup for notifications to work as intended for apps, including Aqua Mail. To ensure the best possible notification delivery check the following options:
Android Settings > Notifications & Status Bar > Manage notifications > Aqua Mail > Allow notifications
Android Settings > Battery > App battery management > Aqua Mail > Allow foreground activity
Android Settings > Battery > App battery management > Aqua Mail > Allow background activity
Android Settings > Battery > App battery management > Aqua Mail > Allow Auto-launch
Android Settings > Battery > More battery settings > Optimise battery use > Aqua Mail > Don't optimise
Recent apps menu > Aqua Mail > Options > Lock
Guide created with ColorOS 11.1 / Android 11. Please note some setting names and locations may differ depending on your ColorOS version and device model.