Unlike many people I like the status bar icon - shows Aquamail is running and checking for new mail.
I have noted that when there is no data connection, the status bar icon disappears. My suggestion is that rather than the icon disappear, it instead change to red. When there is no icon you don't know if it's not there because Aquamail is not running or some other reason (and I am not getting used to the range of "not doing what it should for some other reason" in all sorts of areas on my very buggy stock ROM; not Aquamail issues but just a very very buggy phone). Show red would indicate Aquamail IS running but cannot communicate with one or more mail servers. Maybe change the standard "All OK" icon to green (I appreciate blue and aqua). even use Green=everything OK, Yellow=some problems (i.e. some accounts OK, some not) and Red=everything terrible (e.g. no data connection).
Not a massive issue, but maybe not a massive change to the app either?