It looks like it has something to do with how your phone's environment is set up. The fact that you observe the same behavior with a different app confirms that.
It has something to do with handling of either specific intent, or intents in general.
Kostya might have some additional insights (but again, this is not the issue of Aquamail).
If you provide your Android version (any non-standard ROM?) and phone model, that information might be helpful.
I'd expect that you might get more detailed help in a forum/ from tech support that is specific to your phone (or the ROM, if is non-standard).