That behavior explained changed.
If you use multiple accounts (I have 4) and use smart folder as default on opening AquaMail you'll understand. If not, it's waist of time.
I attached the old behavior.
This change is annoying me too, and I can't find an option to disable it, so I came here specifically to check if it was an already-reported bug.
I use the Smart Folder View as my app startup location for AquaMail, so single-pressing on the Back button from that same location should
exit back out of the app.
I rarely want to see the full accounts list - I only have three email accounts, and they are each colour-coded in the Smart View page anyway - and if I ever do want to then there's already a link at the top of the screen that takes me there.
Also, now if I do happen to go to the Accounts page from my initial Smart Folder view, and then press Back, it exits the app rather than going back to the previous page (i.e. returning to the Smart View as it should).
The Back button should
always take you back to the last viewed location, it should not go forward to a new page. There's a clue in the name of the button.
Andre