Oh, OK, I had assumed that Aquamail would prefer to use itself for e-mail sending. But, on second thought, it makes sense that the user should be able to choose what the system does (and it wouldn't make sense for Aquamail to duplicate a setting that already exists in the system).
I wouldn't mind a pop-up, the first time you tap an e-mail address, saying "Aquamail is not your default e-mail app. Would you like to change it now?"
Thanks.
James