I had the same problem, nr of unread mails did not show on my homescreen icon. Fortunately, the aqua mail support people answered my call for help quickly and gave me the following tip. It has solved the issue for me (Samsung tab S3):
Here's what they told me:
Starting with Android 8.0 (Oreo) Samsung completely changed how the “unread badge” feature works. Previously an app could directly set the badge’s value, this is not longer possible. At all.
The “new way” is managed by the phone’s software and its value is automatically taken from the app’s notifications (in the phone’s top bar).
There are two settings to check / adjust before this works.
1 – Phone settings -> Apps -> Aqua Mail -> Notifications -> enable “Show badge”.
This however will only trigger on single-message notification, so you’ll have either nothing or a “1” on the badge.
To make it work for multiple-message notifications, please:
2 – Aqua Mail settings -> message notifications -> enable “Smart Watch enhancements”.
This also changes how on-phone notifications look. You’ll be able to expand (swipe down) on a notification that’s “about” more than one message, turning into individual “cards”, one per message, each with its down action buttons (mark read and so on).
This “enhanced” notification style – which first appeared in Android 7.0 – works with the new Samsung badge feature as expected, even when there are multiple unread messages.
PS – the badge count will clear when the app’s notifications are cleared.
