Same happens when you open conversations and then open a new message. when you sended it, and go back you wont go back to the conversations but actually going to the multitask screen. Its indeed anoying and dont understand why nokia made it this way.

Going back with email does not let every screen go back to the multitask but switches back to the previous page. this is how phone and conversations should work.

By the way this falling back to multitask screen happens with allot of apps when pushing the previous button.