Samsung's One UI requires additional setup for notifications to work as intended for apps, including Aqua Mail. To ensure the best possible notification delivery check the following options:

Settings > Notifications > See All (if needed) > Aqua Mail > Enable
Then tap on Aqua Mail > Enable "App icon badges"
General notifications tab > in the next window manage the style notification specifics
Settings > Device care > Battery > Options (this will appear as three dots in the top right corner) > Settings > Sleeping apps > Ensure Aqua Mail is not on this list.

Recent apps > Aqua Mail > Options > Lock

Additionally, make sure to exclude Aqua Mail from any battery optimizer that may be running and configure the app's priority in your settings.