First, replying or forwarding uses the account where the original message was received.
Second, if you'd like to set a specific account as the default for new messages -- that's under app settings -> composing and sending
Third, the above will only apply when you click "new message" from the app's main window (account list) or the smart folder.
When you click "new message" from inside a particular folder's message list, then the app will use the respective account (the one which "owns" the folder you're in).